Most recent kernel where this bug did *NOT* occur:
2.6.20.8

Distribution:
Fedora Core 6

Hardware Environment:
Sony Vaio 2.4gh/512 mb
00:00.0 Host bridge: Silicon Integrated Systems [SiS] 645xx (rev 03)
00:01.0 PCI bridge: Silicon Integrated Systems [SiS] SG86C202
00:02.0 ISA bridge: Silicon Integrated Systems [SiS] SiS963 [MuTIOL Media IO]
(rev 14)
00:02.1 SMBus: Silicon Integrated Systems [SiS] SiS961/2 SMBus Controller
00:02.5 IDE interface: Silicon Integrated Systems [SiS] 5513 [IDE]
00:02.6 Modem: Silicon Integrated Systems [SiS] AC'97 Modem Controller (rev a0)
00:02.7 Multimedia audio controller: Silicon Integrated Systems [SiS] AC'97
Sound Controller (rev a0)
00:03.0 USB Controller: Silicon Integrated Systems [SiS] USB 1.0 Controller (rev 0f)
00:03.1 USB Controller: Silicon Integrated Systems [SiS] USB 1.0 Controller (rev 0f)
00:03.2 USB Controller: Silicon Integrated Systems [SiS] USB 1.0 Controller (rev 0f)
00:03.3 USB Controller: Silicon Integrated Systems [SiS] USB 2.0 Controller
00:04.0 Ethernet controller: Silicon Integrated Systems [SiS] SiS900 PCI Fast
Ethernet (rev 90)
00:07.0 Ethernet controller: Atheros Communications, Inc. AR5212 802.11abg NIC
(rev 01)
00:0a.0 CardBus bridge: Ricoh Co Ltd RL5c476 II (rev aa)
00:0a.1 CardBus bridge: Ricoh Co Ltd RL5c476 II (rev aa)
00:0a.2 FireWire (IEEE 1394): Ricoh Co Ltd R5C552 IEEE 1394 Controller (rev 02)
01:00.0 VGA compatible controller: nVidia Corporation NV17 [GeForce4 420 Go]
(rev a3)
Software Environment:

KDE.
Kernel compiled with full debugging
Standard nv driver - NOT nvidia.

Problem Description:

Kernel boots. X starts. Hard lock occurs immediately on launching an
application. Complete freeze  stops logging (no panic or error messages. No
debug information whatsoever. Neither SysReq nor alt-ctl-del are responsive.

Steps to reproduce:

Boot into X. Launch any application (web, mail client, etc).