The only possible change I can imagine for the 15" MBP is a matte screen, and/or a built-in battery like the 17" MBPs (although a bit smaller due to the smaller size of the 15" MBP). However, I don't know if that will happen at all. I think they may give users the matte screen, but not the battery. If Apple wanted to give the 15" MBP the built-in battery, they probably could have done it when it was introduced. It's not like Apple only thought of this idea in November 2008, after the 15" MBP was already released.
With regards to the specs, I don't think it matters if they speedbump it a bit. That's all you're likely to get next time anyway. I think the real change will come when they start using Intel's Arrandale processor, which may be in early-ish 2010. Who knows.