I've had to do this for several of these laptops now so I thought I'd just make a picture tutorial to explain. It's been pretty much the same for all models T60 and greater. What we've been doing is upgrading machines with either 1 or 2GB up to 4GB. The memory is located under the touchpad on the front panel, which is removable via long screws on the bottom. All you need is a philips head precision screwdriver and your new ram, which must be compatible with the machine.
First off, you will need a precision screwdriver just like this:
Start by flipping the laptop over. Find the holes that have a little etched memory-chip icon beside them. Some are for more than just the ram, but there will be 5 that have the memory-chip beside them and they will be towards the front of the laptop (away from the battery).
Unscrew the screws using the precision driver and put them aside. Some may not come out easily but once you flip the laptop back over they'll fall so make sure to do it over a secure area so you don't lose them.
Flip the laptop back to it's normal orientation and open the lid. You should be able to grab the front panel and lift it up by the corners.
Flip the lid up but be careful not to pull too hard on the ribbon cable connecting the touchpad. Right underneath you will see the ram that is currently installed.
Pull outwards - away from the ram module on the little metal tabs holding it in. It shouldn't require a lot of effort and the ram will pop up if you've done it right.
It should slide easily out. Reverse the steps to install the new ram. Here is the new ram installed in my T61p:
After that is done, make sure to boot the laptop up to see that all is working correctly and your ram is found. Remember - if you're running a 32bit OS, you'll only get to use around 3GB of RAM. You must run a 64bit OS to use all 4GB.
The front panel will clip back down with a little gentle rocking action and the screws should go back in to be thumb-tight.
Update 9/26/2008 - I upgraded my T61p to 8GB of RAM a few months back and it works perfectly. I used G.Skill memory and have had no issues. This is with Vista 64.