Mid-2007 Santa Rosa 2,2 Core 2 Duo here.
I had the Logic Board replaced almost two years ago under warranty, and I upgraded as soon as I got it to 4GB of RAM and later 320GB HDD. Not planning to upgrade until this thing dies or if a new OS appears and it's both
a)not supported on my MBP
b)has a killer feature
As I get more and more into 3D stuff, I might get a PC laptop if I really need more processing power on the go, but I doubt it, and in any case, the MBP would go to my girlfriend, who wouldn't need a new OS anyway.
If the hard drive dies, I'll swap it, so it's all down to the screen and the logic board, which are the two components too expensive to be worth replacing.
So basically, until it dies.