The newer model is faster, has a tilting "head' so he can follow your movements, and works with an iPhone 4, iPhone 4S, or iPod touch 4th-generation as his brain. Any other iOS device can control him, so Romo's the perfect place to reuse an previous-generation iPhone or iPod touch if you want to get a new one. There's a telepresence mode that works with the iPhone 4S and is useful if you want others to be able to control Romo remotely for work or play.
If you missed any of the earlier info about Romo, here's a TED Talk video featuring Romotive founder and CEO Keller Rinaudo introducing Romo to a crowd as well as a promotional video from the company's website. We're attempting to get a visit from Romo for a full TUAW review in the near future.