I upgraded my ThinkPad T60 (1953-D7U) from a Core Duo to a Core 2 Duo. It was a simple upgrade (granted you are familiar with taking all of your laptop apart). I was switching from Fedora 7 32-bit to Fedora 8 64-bit, plus I wanted the doubled cache (4MB) and the extras that come with it. I went from a T2300 to a T7200 667MHZ. I got it for $80 cheaper on Ebay (lucky snipe) than what NewEgg.com had it for!
I definitely think it was a worthwhile because I did notice a decent performance boost on my media encoding, Games, file extraction and compression...etc
HOWEVER, things to keep in mind:
1- This might probably void your warranty even if your laptop doesn't get damaged in the process or due to the upgrade.
Let's just say after you do the upgrade, maybe you might want to keep the older CPU hanging around...to ward off evil spirits
2- You need to upgrade your T60 to the latest BIOS version available before you upgrade...why? Well it's because Core 2 Duo support comes with later BIOS versions (I think that's 2.01 and above ?)