Ruby on Rails
PHP
Java
Perl
That's not all of the technologies used, but I think that's a pretty good representation of the mix in order of popularity (there might be a Python group in there, too, but I can't think of who). One group is writing their own language and framework (Tsumobi, implemented in J2ME), while others are working predominantly in JavaScript (Zenter) or ActionScript (Heysan) for large parts of their product and the backend language is entirely unimportant (I'm betting Zenter probably only has a couple thousand lines of server-side code, versus tens of thousands of lines of JavaScript). Actually, nearly everyone is doing some of their work in JavaScript, and it's the one language that I know I can ask any of the developers in the program questions about.
Cheers, Ralph.
Python is probably second most common. There are also some using Java, because they have to for phones.
That said, real startup programmers hack in Python.
I might actually give Rails a try on my next short consulting gig. All these YCers can't all be wrong.