I really got into GTA IV at the end of the fall semester and noticed that Rockstar had nothing on their website about a new one, hopefully they are working on the new one. I agree with Duckman on Vice City, it is def my favorite.
We'll probably hear something at E3. I know they'll be pushing LA Noire for the near future, even though they're only publishing. The only other thing I know they're working on is Agent. I hope that whenever they do release it that it's set in a new location.
Vice City and San Andreas are very close for me. Both did a great job of capturing a certain feel for the time period they were set in. Both had great soundtracks as well. I'll probably have to give the nod to San Andreas as my favorite because the map was so huge you could literally never run out of things to do, and the fact that you didn't instantly die the second you fell in the water unlike the previous two games.
I hope they try to set the next GTA in a different time period because to me that's what really stood out about the best games in the series.