This is the same question I had in my mind also. My main complaint is application loading times and delays. I went to the Apple Store the other day and tried out one of those 2.66 GHZ i7 Macbook Pros. I didn't see a significant speed difference in loading and using applications like iPhoto and Aperature on the new Macbook Pro versus my current one. This is why I am leaning towards an SSD instead of upgrading to a new model. I have the late 2008 unibody 2.4 GHZ Macbook Pro. I'm sure the i7 is much faster in things like video encoding but I don't do that much in my macbook.
I've been reading good things about the OWC Mercury Extreme Pro SSD. The only caveats is the sleep issue (common with Sandforce based SSD's) and it seems that some people may be having an issue with the latest firmware. I also don't like that there isn't a Mac SSD firmware update tool for any of these SSDs. I will have to think abt it for a while. I recently bought a Crucial C300 128GB for my PC and it really is blazing fast and was plug and play to set up. I would stay with that brand but it works best with an OS which supports TRIM (which Windows 7 does but Mac OS X doesn't).
I got a 2.66 i7 MBP like one month ago and updated the HDD to a 500 Gb 7200 rpm one (from the 5400 rpm that came with it). My previous computer, which I still have is an early 2008 2.4 C2D penryn MBP with a 5400 rpm 200 Gb HDD. Both are running up to date Mac OSX 10.6.6 and both have 4 Gb of RAM which is enough for all tasks I normally perform with them.
The thing is that although the ne one is faster than the previous 3 year old one, the difference is really small. Maybe it turns on much faster, but I restart the computer like once every two months, so that means nothing to me. I'm really surprised how all these incredible advances are announced every two months and how obsolete your computer is within weeks, but at the end of the day, the difference from the top of the line brand new MBP to the entry level-3 year old- MBP is like one second or less when opening iPhoto.