I tested this with the Jaunty beta, and it is a problem with a motherboard here:

shane@shane-desktop:~$ uname -a
Linux shane-desktop 2.6.28-11-generic #41-Ubuntu SMP Wed Apr 8 04:39:23 UTC 2009 x86_64 GNU/Linux
shane@shane-desktop:~$ cat /proc/cpuinfo 
processor	: 0
vendor_id	: AuthenticAMD
cpu family	: 15
model		: 107
model name	: AMD Athlon(tm) 64 X2 Dual Core Processor 4400+
stepping	: 2
cpu MHz		: 1000.000
cache size	: 512 KB
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 2
apicid		: 0
initial apicid	: 0
fpu		: yes
fpu_exception	: yes
cpuid level	: 1
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow rep_good pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy 3dnowprefetch
bogomips	: 1999.79
TLB size	: 1024 4K pages
clflush size	: 64
cache_alignment	: 64
address sizes	: 40 bits physical, 48 bits virtual
power management: ts fid vid ttp tm stc 100mhzsteps

processor	: 1
vendor_id	: AuthenticAMD
cpu family	: 15
model		: 107
model name	: AMD Athlon(tm) 64 X2 Dual Core Processor 4400+
stepping	: 2
cpu MHz		: 1000.000
cache size	: 512 KB
physical id	: 0
siblings	: 2
core id		: 1
cpu cores	: 2
apicid		: 1
initial apicid	: 1
fpu		: yes
fpu_exception	: yes
cpuid level	: 1
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow rep_good pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy 3dnowprefetch
bogomips	: 1999.79
TLB size	: 1024 4K pages
clflush size	: 64
cache_alignment	: 64
address sizes	: 40 bits physical, 48 bits virtual
power management: ts fid vid ttp tm stc 100mhzsteps

shane@shane-desktop:~$ lspci
00:00.0 Host bridge: VIA Technologies, Inc. K8M890CE Host Bridge
00:00.1 Host bridge: VIA Technologies, Inc. K8M890CE Host Bridge
00:00.2 Host bridge: VIA Technologies, Inc. K8M890CE Host Bridge
00:00.3 Host bridge: VIA Technologies, Inc. K8M890CE Host Bridge
00:00.4 Host bridge: VIA Technologies, Inc. K8M890CE Host Bridge
00:00.5 PIC: VIA Technologies, Inc. K8M890CE I/O APIC Interrupt Controller
00:00.7 Host bridge: VIA Technologies, Inc. K8M890CE Host Bridge
00:01.0 PCI bridge: VIA Technologies, Inc. VT8237 PCI bridge [K8T800/K8T890 South]
00:02.0 PCI bridge: VIA Technologies, Inc. K8T890 PCI to PCI Bridge Controller
00:03.0 PCI bridge: VIA Technologies, Inc. K8T890 PCI to PCI Bridge Controller
00:0f.0 IDE interface: VIA Technologies, Inc. VT8237A SATA 2-Port Controller (rev 80)
00:0f.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A/B/VT823x/A/C PIPC Bus Master IDE (rev 07)
00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.1 Controller (rev a0)
00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.1 Controller (rev a0)
00:10.2 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.1 Controller (rev a0)
00:10.3 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.1 Controller (rev a0)
00:10.4 USB Controller: VIA Technologies, Inc. USB 2.0 (rev 86)
00:11.0 ISA bridge: VIA Technologies, Inc. VT8237A PCI to ISA Bridge
00:11.7 Host bridge: VIA Technologies, Inc. VT8251 Ultra VLINK Controller
00:12.0 Ethernet controller: VIA Technologies, Inc. VT6102 [Rhine-II] (rev 7c)
00:13.0 Host bridge: VIA Technologies, Inc. VT8237A Host Bridge
00:13.1 PCI bridge: VIA Technologies, Inc. VT8237A PCI to PCI Bridge
00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] HyperTransport Technology Configuration
00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Address Map
00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] DRAM Controller
00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Miscellaneous Control
02:00.0 VGA compatible controller: nVidia Corporation G94 [GeForce 9600 GT] (rev a1)
04:05.0 Multimedia audio controller: Creative Labs SB Live! EMU10k1 (rev 06)
04:05.1 Input device controller: Creative Labs SB Live! Game Port (rev 06)
shane@shane-desktop:~$ sensors
k8temp-pci-00c3
Adapter: PCI adapter
Core0 Temp:  +12.0°C                                    
Core0 Temp:  -10.0°C                                    
Core1 Temp:   -3.0°C                                    
Core1 Temp:   -8.0°C