I believe it depends on the school. I know UIC will accept a letter from a TA, but you'd probably want one from a prof in addition to that. If you go to a massive school like me (U of Illinois Champaign-Urbana), they may be a bit more forgiving, because not EVERYONE is able to get to know a prof. But really, you know that having a letter from a prof will make you look that much more impressive. HOWEVER, if it's between a stellar letter from a TA and a ho-hum letter from a prof, well, it may be better to get the letter from the TA. So basically, use your best judgment. I'm in the same boat, so I feel ya.. good luck!