How do you like this one: http://students.mimuw.edu.pl/~ksoltys/tcs.gif ? When you draw this little line between sigma and the infinity symbol, you get a wind-up sigma ;) which I think is a nice symbol for TCS. I used this logo for TCS students' club in my department, but I think it wouldn't conflict.
sigma http://students.mimuw.edu.pl/%7Eksoltys/tcs.gif
edit: I made some changes: sigma is now grey and it has two gears instead of the infinity symbol. I didn't change the sigma's font: sans serif fonts deprive it of it's serious mathematical feel, and all serif ones I tried force it to be large (otherwise serifs overlap with the "t"). If someone knows some better font for the sigma, let me know, I will change it. You may also tweak the (GIMP) source: http://students.mimuw.edu.pl/~ksoltys/tcs2.xcf
