I can confirm this bug too... I thought it was a single-key iussue (and then hardware), but it isn't... In fact playing @ an FPS (and thus using WASD keys) the W key was pretty allways in the 'pressed state'... trying to test the same key using arrow keys, the [UP] key automagically 'binded' to down-state. Also playing on WINE it's the same thing, an I suspect that if we press any key for 10-20 seconds on the desktop the problem will be the same. So I think it's a xserver problem, and a very annoying one...

elegos@medea:~$ uname -a
Linux medea 2.6.24-8-generic #1 SMP Thu Feb 14 20:40:45 UTC 2008 i686 GNU/Linux
elegos@medea:~$ lspci | grep nVidia
01:00.0 VGA compatible controller: nVidia Corporation GeForce 8400M GS (rev a1)
elegos@medea:~$ apt-show-versions | grep xorg
xserver-xorg-input-kbd/hardy uptodate 1:1.2.2-3ubuntu1
xserver-xorg-video-s3/hardy uptodate 1:0.5.0-4
xserver-xorg/hardy uptodate 1:7.3+10ubuntu4
xserver-xorg-video-voodoo/hardy uptodate 1:1.1.1-5
xserver-xorg-video-vga/hardy uptodate 1:4.1.0-8
xserver-xorg-video-cirrus/hardy uptodate 1:1.1.0-8
xserver-xorg-video-newport/hardy uptodate 1:0.2.1-4ubuntu1
xserver-xorg-input-synaptics/hardy uptodate 0.14.7~git20070706-1ubuntu3
xserver-xorg-video-intel/hardy uptodate 2:2.2.0.90-2ubuntu2
xserver-xorg-video-mga/hardy uptodate 1:1.4.8.dfsg.1-1
xserver-xorg-video-tseng/hardy uptodate 1:1.1.1-4
xserver-xorg-video-ati/hardy uptodate 1:6.7.197-1ubuntu1
xserver-xorg-video-rendition/hardy uptodate 1:4.1.3.dfsg.1-4
xorg/hardy uptodate 1:7.3+10ubuntu4
xserver-xorg-video-nsc/hardy uptodate 1:2.8.3-2
xserver-xorg-input-evdev/hardy uptodate 1:1.2.0-1
xserver-xorg-video-i740/hardy uptodate 1:1.1.0-7
xserver-xorg-input-vmmouse/hardy uptodate 1:12.4.3-1ubuntu1
xserver-xorg-video-fbdev/hardy uptodate 1:0.3.1-4
xserver-xorg-core/hardy uptodate 2:1.4.1~git20080131-1ubuntu2
xserver-xorg-input-all/hardy uptodate 1:7.3+10ubuntu4
xserver-xorg-video-cyrix/hardy uptodate 1:1.1.0-8
xserver-xorg-video-openchrome/hardy uptodate 1:0.2.901-0ubuntu4
xserver-xorg-video-sis/hardy uptodate 1:0.9.3-6
xserver-xorg-video-glint/hardy uptodate 1:1.1.1-8
xserver-xorg-video-vmware/hardy uptodate 1:10.15.2-1
xserver-xorg-video-all/hardy uptodate 1:7.3+10ubuntu4
xserver-xorg-video-dummy/hardy uptodate 1:0.2.0-7
xserver-xorg-video-trident/hardy uptodate 1:1.2.3-6
xserver-xorg-video-savage/hardy uptodate 1:2.1.3-5
xserver-xorg-video-v4l/hardy uptodate 1:0.1.1-6ubuntu1
xserver-xorg-video-neomagic/hardy uptodate 1:1.1.1-8
xserver-xorg-video-imstt/hardy uptodate 1:1.1.0-7
xserver-xorg-input-wacom/hardy uptodate 1:0.7.9.3-2ubuntu2
xserver-xorg-video-sisusb/hardy uptodate 1:0.8.1-9
xserver-xorg-video-vesa/hardy uptodate 1:1.3.0-4ubuntu3
xserver-xorg-video-nv/hardy uptodate 1:2.1.7-1
xserver-xorg-video-s3virge/hardy uptodate 1:1.9.1-7
xserver-xorg-video-tga/hardy uptodate 1:1.1.0-9ubuntu1
xserver-xorg-video-ark/hardy uptodate 1:0.6.0-9
xserver-xorg-video-i128/hardy uptodate 1:1.2.1-4
xserver-xorg-input-mouse/hardy uptodate 1:1.2.3-2
xserver-xorg-video-via/hardy uptodate 1:0.2.2-5
xserver-xorg-video-tdfx/hardy uptodate 1:1.3.0-6
xserver-xorg-video-apm/hardy uptodate 1:1.1.1-10
xserver-xorg-video-psb/hardy uptodate 0.2.1-1ubuntu3
xserver-xorg-video-i810/hardy uptodate 2:1.7.4-0ubuntu7
xserver-xorg-video-amd/hardy uptodate 2.7.7.6-1
xserver-xorg-video-siliconmotion/hardy uptodate 1:1.5.1-3
xserver-xorg-video-chips/hardy uptodate 1:1.1.1-9