Hi,
I've been reading up on this as I am looking at getting a 13" macbook pro and came across an article that said that 13" Macbooks and Macbook Pros don't have the physical space on the logic board for the GPU that would be required:
http://arstechnica.com/apple/news/2010/04/why-the-13-macbook-pro-didnt-get-a-core-i5-upgrade.ars