I've been having this problem for the last couple of days. I don't remember updating or changing anything in particular that could have triggered this problem.
My system sometimes randomly locks up and the only way out is to REISUB. Examining the Xorg logs suggests that Xorg gets stuck in an infinite loop with these error messages.
[ 9109.996] (WW) NVIDIA(0): WAIT (2, 6, 0x8000, 0x00004da8, 0x00007c80)
[ 9116.996] (WW) NVIDIA(0): WAIT (1, 6, 0x8000, 0x000050a8, 0x00007c80)
[ 9120.004] (WW) NVIDIA(0): WAIT (2, 7, 0x8000, 0x000050a8, 0x00000a68)
[ 9127.004] (WW) NVIDIA(0): WAIT (1, 7, 0x8000, 0x000050a8, 0x00000a68)
[ 9217.861] [mi] EQ overflowing. The server is probably stuck in an infinite loop.
I run Arch Linux, KDE 4.7.0, Kernel 3.0.3 with BFS scheduler and 280.13 drivers. I tried 285.03 too but the problem stays.
The hardware is GF 9800M GTS, my CPU is Core 2T9550.
I'll try downgrading to 2.6.39 as I'm sure everything worked fine with it.