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][1]


  [1]: https://i.sstatic.net/whNQ9.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 source: http://students.mimuw.edu.pl/~ksoltys/tcs2.xcf

![grey][2]


  [2]: http://students.mimuw.edu.pl/~ksoltys/tcs2.gif