I have the same problem with the same Laptop under breezy, IBM Thinkpad T40p:
ksoftirqd/0 takes up all the processing power, the processor goes up to 1,6 GHz.

I'm using Kernel 2.6.12-9-386.

It seems to be related to switching of Wifi, with the Atheros module still
loaded. It happens using the Fn+F5 Switch or doing 
sudo echo -n 3 > /sys/class/net/ath0/device/power/state
in console.
It is enough to switch the Wifi card back on to let the precesor state go back down.

ras@Nero:~$ lspci
0000:00:00.0 Host bridge: Intel Corp. 82855PM Processor to I/O Controller (rev 0 3)
0000:00:01.0 PCI bridge: Intel Corp. 82855PM Processor to AGP Controller (rev 03 )
0000:00:1d.0 USB Controller: Intel Corp. 82801DB/DBL/DBM (ICH4/ICH4-L/ICH4-M) US
B UHCI Controller #1 (rev 01)
0000:00:1d.1 USB Controller: Intel Corp. 82801DB/DBL/DBM (ICH4/ICH4-L/ICH4-M) US
B UHCI Controller #2 (rev 01)
0000:00:1d.2 USB Controller: Intel Corp. 82801DB/DBL/DBM (ICH4/ICH4-L/ICH4-M) US
B UHCI Controller #3 (rev 01)
0000:00:1d.7 USB Controller: Intel Corp. 82801DB/DBM (ICH4/ICH4-M) USB 2.0 EHCI
Controller (rev 01)
0000:00:1e.0 PCI bridge: Intel Corp. 82801 PCI Bridge (rev 81)
0000:00:1f.0 ISA bridge: Intel Corp. 82801DBM LPC Interface Controller (rev 01)
0000:00:1f.1 IDE interface: Intel Corp. 82801DBM (ICH4) Ultra ATA Storage Contro
ller (rev 01)
0000:00:1f.3 SMBus: Intel Corp. 82801DB/DBL/DBM (ICH4/ICH4-L/ICH4-M) SMBus Contr
oller (rev 01)
0000:00:1f.5 Multimedia audio controller: Intel Corp. 82801DB/DBL/DBM (ICH4/ICH4
-L/ICH4-M) AC'97 Audio Controller (rev 01)
0000:00:1f.6 Modem: Intel Corp. 82801DB/DBL/DBM (ICH4/ICH4-L/ICH4-M) AC'97 Modem
 Controller (rev 01)
0000:01:00.0 VGA compatible controller: ATI Technologies Inc Radeon R250 Lf [Rad
eon Mobility 9000 M9] (rev 02)
0000:02:00.0 CardBus bridge: Texas Instruments PCI1520 PC card Cardbus Controlle
r (rev 01)
0000:02:00.1 CardBus bridge: Texas Instruments PCI1520 PC card Cardbus Controlle
r (rev 01)
0000:02:01.0 Ethernet controller: Intel Corp. 82540EP Gigabit Ethernet Controlle
r (Mobile) (rev 03)
0000:02:02.0 Ethernet controller: Atheros Communications, Inc. AR5211 802.11ab N
IC (rev 01)
ras@Nero:~$ lsmod
Module                  Size  Used by
binfmt_misc            10888  1
rfcomm                 34972  0
l2cap                  22404  5 rfcomm
ipv6                  217408  8
speedstep_centrino      7380  1
cpufreq_userspace       4444  1
cpufreq_stats           5124  0
freq_table              4484  2 speedstep_centrino,cpufreq_stats
cpufreq_powersave       1920  0
cpufreq_ondemand        5916  0
cpufreq_conservative     6820  0
pcmcia                 24584  4
radeon                 68352  1
drm                    58004  2 radeon
video                  16004  0
tc1100_wmi              6916  0
sony_acpi               5516  0
pcc_acpi               11392  0
ibm_acpi               17908  0
hotkey                  9508  0
dev_acpi               11396  0
i2c_acpi_ec             5760  0
i2c_core               19728  1 i2c_acpi_ec
button                  6672  0
battery                 9604  0
container               4608  0
ac                      4996  0
af_packet              20232  2
wlan_wep                5888  1
irtty_sir               7808  0
sir_dev                17324  1 irtty_sir
irda                  159804  2 irtty_sir,sir_dev
crc_ccitt               2176  1 irda
floppy                 52692  0
rtc                    11832  0
pcspkr                  3652  0
hci_usb                13192  0
bluetooth              43012  5 rfcomm,l2cap,hci_usb
ath_pci                69148  0
ath_rate_sample        14344  1 ath_pci
wlan                  120988  4 wlan_wep,ath_pci,ath_rate_sample
ath_hal               148432  3 ath_pci,ath_rate_sample
yenta_socket           22540  2
rsrc_nonstatic         12032  1 yenta_socket
pcmcia_core            44932  3 pcmcia,yenta_socket,rsrc_nonstatic
snd_intel8x0           30144  1
snd_ac97_codec         72188  1 snd_intel8x0
snd_pcm_oss            46368  0
snd_mixer_oss          16128  1 snd_pcm_oss
snd_pcm                78344  3 snd_intel8x0,snd_ac97_codec,snd_pcm_oss
snd_timer              21764  1 snd_pcm
snd                    48644  8 snd_intel8x0,snd_ac97_codec,snd_pcm_oss,snd_mixe
r_oss,snd_pcm,snd_timer
soundcore               9184  1 snd
snd_page_alloc         10120  2 snd_intel8x0,snd_pcm
tpm_atmel               5504  0
tpm_nsc                 6528  0
tpm                     9504  2 tpm_atmel,tpm_nsc
hw_random               5268  0
shpchp                 80612  0
pci_hotplug            24628  1 shpchp
intel_agp              21276  1
agpgart                32328  2 drm,intel_agp
dm_mod                 50364  1
joydev                  9280  0
tsdev                   7616  0
evdev                   9088  1
psmouse                26116  0
mousedev               10912  1
parport_pc             31812  1
lp                     11460  0
parport                32072  2 parport_pc,lp
md                     40656  0
ext3                  115976  2
jbd                    48536  1 ext3
thermal                13192  0
processor              23100  2 speedstep_centrino,thermal
fan                     4740  0
e1000                  89780  0
ehci_hcd               29448  0
uhci_hcd               28048  0
usbcore               104188  4 hci_usb,ehci_hcd,uhci_hcd
ide_cd                 36996  0
cdrom                  33952  1 ide_cd
ide_disk               16128  4
ide_generic             1664  0
piix                    9476  1
ide_core              125268  4 ide_cd,ide_disk,ide_generic,piix
unix                   24624  882
vesafb                  8088  0
capability              5000  0
commoncap               6784  1 capability
vga16fb                12232  1
vgastate                8320  1 vga16fb
softcursor              2432  2 vesafb,vga16fb
cfbimgblt               2944  2 vesafb,vga16fb
cfbfillrect             3840  2 vesafb,vga16fb
cfbcopyarea             4480  2 vesafb,vga16fb
fbcon                  34176  72
tileblit                2560  1 fbcon
font                    8448  1 fbcon
bitblit                 5248  1 fbcon