I picked up my current Macbook Pro in June, and I plan on keeping it for 5 years+. The last computer I had I purchased in 2001, and I was still using it, with only adding in more ram and a new hard drive. The only reason why I purchased a Macbook Pro was because I wanted something portable, and I wanted to switch to Mac.
I'm not a big gamer, and I can't imagine the standard apps (Mail, iChat, iCal, iTunes, Safari, etc.), Office, or anything else becoming too bloated for my current computer anytime soon. My computational and programming apps (Matlab, Eclipse,) might take a little longer to compile/compute, but they should still be perfectly functional. When needed, I'll probably bump up the ram to 4GB but the internals should remain usable as long as nothing breaks.