Dunno if this works for everyone else, but I've had a lot of luck closing it from outside the program. Save the game (Y), don't quit (N), alt-tab or what-have-you, then kill it from the menu bar with a right-click.
Might just be an xfce thing (Kubuntu OS using an xfce front-end), but I usually find wrestling the mouse back means either going to another desktop or having another window to alt-tab to from Wasteland on the same desktop. A minor setback.

Edit: Y'know what else nets me a kernel panic crash? When my whole party is killed by a shadowclaw and I lose the game.
If you see the skull and you've had this problem before, don't hit Enter. It tries to quit the game. Danger, Will Robinson, danger. :T

Edit: This is NOT a 100% solution, I just find I have better luck and few kernel panics this way.

Alternatively, you could just install the new patch, which seems to do away with the problem entirely.

Addendum: Okay, so after a great many successful and safe quits, Kubuntu froze just now on a Wasteland save/quit. Aw well. Still way more stable than it was.