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If the /home partition fills up, gimp will choke trying to write to the swap file in ~/.gimp. No surprise there, but it appears to get stuck in a loop reporting the error. Hundreds of dialog boxes pop up reporting the write error. X is unresponsive at this point. There's no way to stop it except switching to a console & killing the gimp process. Sadly, when I did this the X server did not recover. The gimp windows all closed, but the server was so tweaked it could not be shut down. The console hot-keys no longer worked, nor did ctrl-alt-backspace. It took a ctrl-alt-del to get the machine out of this state. This seems to indicate a problem in the X server as well (S3 ViRGE, SVGA server).
The fix would require significant changes to the GIMP tile manager. I don't expect for this problem to be addressed until version 1.2 of The GIMP. The GIMP developers were aware of a problem in 1.0.x series. Hopefully 1.2 will work better - it has significant tile manager improvements.