Switching to nv driver solves the problem.

Switching to xinerama solves the problem.

Removing the second screen resolves the problem.

Sigh.  I'll file a bug with nvidia about it :(

FWIW, I've attached my xorg.conf (which is pretty messy) and the output of lspci -vv is:

00:00.0 Host bridge: VIA Technologies, Inc. VT8377 [KT400/KT600 AGP] Host Bridge
        Subsystem: ASUSTeK Computer Inc. A7V8X motherboard
        Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz+ UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort+ >SERR- <PERR-
        Latency: 0
        Region 0: Memory at f0000000 (32-bit, prefetchable) [size=128M]
        Capabilities: [80] AGP version 3.5
                Status: RQ=32 Iso- ArqSz=0 Cal=2 SBA+ ITACoh- GART64- HTrans- 64bit- FW- AGP3+ Rate=x4,x8
                Command: RQ=32 ArqSz=0 Cal=2 SBA+ AGP+ GART64- 64bit- FW- Rate=x8
        Capabilities: [c0] Power Management version 2
                Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:01.0 PCI bridge: VIA Technologies, Inc. VT8235 PCI Bridge (prog-if 00 [Normal decode])
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz+ UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort+ >SERR- <PERR-
        Latency: 0
        Bus: primary=00, secondary=01, subordinate=01, sec-latency=0
        Memory behind bridge: dd000000-dfefffff
        Prefetchable memory behind bridge: dff00000-efffffff
        Secondary status: 66MHz- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- <SERR- <PERR-
        BridgeCtl: Parity- SERR- NoISA- VGA+ MAbort- >Reset- FastB2B-
        Capabilities: [80] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.1 Controller (rev 80) (prog-if 00 [UHCI])
        Subsystem: ASUSTeK Computer Inc. A7V8X-X motherboard
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV+ VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 32, Cache Line Size: 32 bytes
        Interrupt: pin A routed to IRQ 169
        Region 4: I/O ports at d800 [size=32]
        Capabilities: [80] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=375mA PME(D0+,D1+,D2+,D3hot+,D3cold+)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.1 Controller (rev 80) (prog-if 00 [UHCI])
        Subsystem: ASUSTeK Computer Inc. A7V8X-X motherboard
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV+ VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 32, Cache Line Size: 32 bytes
        Interrupt: pin B routed to IRQ 169
        Region 4: I/O ports at d400 [size=32]
        Capabilities: [80] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=375mA PME(D0+,D1+,D2+,D3hot+,D3cold+)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:10.2 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.1 Controller (rev 80) (prog-if 00 [UHCI])
        Subsystem: ASUSTeK Computer Inc. A7V8X-X motherboard
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV+ VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 32, Cache Line Size: 32 bytes
        Interrupt: pin C routed to IRQ 169
        Region 4: I/O ports at d000 [size=32]
        Capabilities: [80] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=375mA PME(D0+,D1+,D2+,D3hot+,D3cold+)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:10.3 USB Controller: VIA Technologies, Inc. USB 2.0 (rev 82) (prog-if 20 [EHCI])
        Subsystem: ASUSTeK Computer Inc. A7V8X-X motherboard rev 1.01
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV+ VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 32, Cache Line Size: 64 bytes
        Interrupt: pin D routed to IRQ 169
        Region 0: Memory at dc800000 (32-bit, non-prefetchable) [size=256]
        Capabilities: [80] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=375mA PME(D0+,D1+,D2+,D3hot+,D3cold+)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:11.0 ISA bridge: VIA Technologies, Inc. VT8235 ISA Bridge
        Subsystem: ASUSTeK Computer Inc. A7V8X-X motherboard
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping+ SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 0
        Capabilities: [c0] Power Management version 2
                Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:11.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A/B/VT823x/A/C PIPC Bus Master IDE (rev 06) (prog-if 8a [Master SecP PriP])
        Subsystem: ASUSTeK Computer Inc. A7V8X-X motherboard rev. 1.01
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 32
        Interrupt: pin A routed to IRQ 255
        Region 4: I/O ports at b800 [size=16]
        Capabilities: [c0] Power Management version 2
                Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:11.5 Multimedia audio controller: VIA Technologies, Inc. VT8233/A/8235/8237 AC97 Audio Controller (rev 50)
        Subsystem: ASUSTeK Computer Inc. A7V8X-X Motherboard
        Control: I/O+ Mem- BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Interrupt: pin C routed to IRQ 193
        Region 0: I/O ports at e000 [size=256]
        Capabilities: [c0] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

00:12.0 Ethernet controller: VIA Technologies, Inc. VT6102 [Rhine-II] (rev 74)
        Subsystem: ASUSTeK Computer Inc. A7V8X-X Motherboard
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV+ VGASnoop- ParErr- Stepping+ SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 32 (750ns min, 2000ns max), Cache Line Size: 32 bytes
        Interrupt: pin A routed to IRQ 185
        Region 0: I/O ports at b400 [size=256]
        Region 1: Memory at dc000000 (32-bit, non-prefetchable) [size=256]
        Capabilities: [40] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0+,D1+,D2+,D3hot+,D3cold+)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

01:00.0 VGA compatible controller: nVidia Corporation NV43 [GeForce 6600/GeForce 6600 GT] (rev a2) (prog-if 00 [VGA])
        Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 248 (1250ns min, 250ns max)
        Interrupt: pin A routed to IRQ 177
        Region 0: Memory at de000000 (32-bit, non-prefetchable) [size=16M]
        Region 1: Memory at e0000000 (32-bit, prefetchable) [size=256M]
        Region 2: Memory at dd000000 (32-bit, non-prefetchable) [size=16M]
        [virtual] Expansion ROM at dffe0000 [disabled] [size=128K]
        Capabilities: [60] Power Management version 2
                Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-
        Capabilities: [44] AGP version 3.0
                Status: RQ=256 Iso- ArqSz=0 Cal=3 SBA+ ITACoh- GART64- HTrans- 64bit- FW+ AGP3+ Rate=x4,x8
                Command: RQ=32 ArqSz=0 Cal=0 SBA+ AGP+ GART64- 64bit- FW- Rate=x8