I'm still using my 2006 MBP Core Duo and plan to wait for the next refresh, whatever that brings, and then I plan to upgrade. My current MBP will then be retired and be integrated into my Ham Radio station, where hopefully it will last another 5 years.
I don't need a laptop with another DVD drive, but I need more oomph than what a MacBook Air gives you (I use my MBP for personal and work -- software development). So when the refresh comes out, I'll get the fastest CPU (or the model with the most cores offered), max out the RAM, and possibly get an SSD. That should last me another 5-6 years.
My advice is to wait one more year unless you really hate using what you have now. We'll either get a major case redesign (hopefully sans DVD drive and with more battery capacity, and probably higher res graphics), or not. At the least it will run faster if that is something you care about.