Force India proved to me that they are really good at improving in 2008, due to the huge amount they made up on the opposition. Progress will slow in 2009 - a combination of the rule changes that Steven mentioned, the fact that progress is more difficult the further up the grid you go and the "sophamore syndrome" problem (which Vijay is showing early symptoms of setting himself up for). However, I still think progress will be made, and with a couple of tenths separating the different positions in the hierarchy, it won't take much progress to steal a place. But the place stolen won't necessarily be from STR or Williams, because rule changes can shake things up considerably.
I'm hoping Force India keeps the entire line-up unchanged. Not only has it proven effective so far, but the last thing you need when going into a major regulation change is to change stuff within the team as well.