My Macbook Pro 17" from June 2009 is going strong and I use it everyday as both a secondary system and a main setup. I also have the latest Lion system on it.
Just upgraded to 8GB RAM and plan on installing a boot SSD to go along side the stock 500 MB. Crucial and Intel SSDs are very expensive. Comparitively, OWC SSDs are supposed to be the best on the market when tested against other brands and also best value. They offer 3Gbps SSDs for your laptop generation (which is the same as mine). I plan on purchasing one.
Granted there is a wall you will hit on a laptop regarding processing speed (since you cannot upgrade a laptop processor unlike current towers) but you can at least maximize your laptop's performance by upgrading RAM (definitely), adding an efficient SSD, proper settings on major memory hogs like Photoshop (if you use it daily) and proper disk/system maintenance. A good SSD being used as a scratch disk would make a difference over a 7200 RPM disk when you factor in long-term degradation of read-write speed in a non-SSD drive.
Regarding your Macbook's longevity, unless you plan of exposing it to abuse, it should last you as long as you want to keep it. My Macbook performs well with no lag in Lion, so it is safe to assume that it will still be productive thru the next couple of OS upgrades.