I used to work at Pivotal and I really enjoyed it. I learned more in a month there than in a year at my previous job. If you're into the idea of TDD and Pair Programming it's incredible. I got to spend time at a bunch of different companies (Mavenlink, Twitter and Get Satisfaction to name a few), and really get a sense for what works in a startup. They have most of the smartest, most pragmatic and nicest programmers I've ever met. I'd highly recommend the place.

If you have any other questions I'd be happy to answer them.