I have a problem that pops up with some games, sometimes:

sauerbraten, lugaru, and nexuiz being the ones that pop to mind.

The problem is that when the game starts/loads the mouse "cursor" in the game will not work... the thing is frozen. The "fix" is to jump to a virtual terminal, via alt-ctrl-1, and restart KDM, then I log back into the session and everything is working swell.

This problem does not occur in Osmos, World of Goo, Warzone 2100, or the Linux Ryzom client.

I thought about adding an explicit /etc/init.d/kdm restart in my /etc/kde4/kdm/Xreset file, but that seems too draconian.

This has been a recurring problem on several machines with several 7000, 8000, and 9000 series Nvidia cards running under the proprietary driver, on both 32-bit and 64-bit AMD processors ever since Lenny and up through Wheezy. And it occurs on the following desktop/windowing environments: KDE4.4, icewm, fluxbox, blackbox, E17.

I would guess that it's a driver issue or a driver+xorg configuration issue.

PS: Please don't suggest that I should use the open source driver.

I have used Ubuntu and Debian with a 9800GT, and also Debian and Slackware with a GTX260. Debian was Lenny, Squeeze (testing at that time) and Sid, with proprietary drivers (195.xx from Testing and Sid, 256.xx from Experimental, 256.xx and higher installed with smxi directly from NVidia). I never had any problem with it.
Are you really sure that this is not a hardware issue? Is there anything in the logs?

I'm pretty sure this isn't a hardware issue, as it's happened under two different processors, three different graphics cards, and only under certain games, and only sometimes. Also, when, for example, sauerbraten, freezes on starting, I can kill the sauer_client process and still run, for example, warzone2100 with no problem, but I'll need to restart KDM before I can run sauerbraten, lugaru, or nexuiz.

What about the logs?

OK, I hit the error again with sauerbraten, killed the process with pkill.
The command line output didn't yield anything of value, and the .xsession-errors also told me nothing, however, /var/log/Xorg.0.log and /var/log/kdm.log may be worth listing:

================================
tail end of /var/log/Xorg.0.log:
================================

the following is repeated several times:
[ 63674.645] [dix] EventToCore: Not implemented yet
[ 63674.645] [dix] DeliverGrabbedEvent. Core conversion failed.
[ 63674.645] [dix] EventToXI2: Not implemented for 15
[ 63674.645] [dix] Virtual core pointer: XI2 conversion failed in DGE (15, 17). Skipping delivery.

and the last lines of the log are:
[ 63695.627] (II) Open ACPI successful (/var/run/acpid.socket)
[ 63695.679] (II) NVIDIA(0): Setting mode "nvidia-auto-select"

=============================
tail end of /var/log/kdm.log:
=============================

the following, as above, is repeated several times:
[dix] Virtual core pointer: XI2 conversion failed in DGE (15, 17). Skipping delivery.
[dix] EventToXI: Not implemented for 15
[dix] Virtual core pointer: XI conversion failed in DGE (15, 17). Skipping delivery.

======================
That's about what I've got for logging