Here's the deal...MBPs are ALWAYS being updated, usually once or twice a year, and chances are less than 6 months after you buy whatever and whenever you buy you won't have the latest and greatest anymore.
That being said, with the chip updates to the i5/i7 processors I'd say any upgrades coming down the pipe before say, early 2011 will probably be incremental or minor price drops.
Go with your gut and buy what and when you need.