Replacing the trackpad turned out to be really easy! Yay!
On my 2009 Macbook Pro 13" Unibody I removed the bottom of the notebook, removed the battery, removed the 8 screws holding the trackpad to the case and disconnected the ribbon wire connecting it to the motherboard, put in new trackpad and reassembled. Took me about 30 minutes. Didn't even need a guide as it was pretty intuitive.
The one part that I got hung up on was that I didn't have a tri-wing screwdriver which you need to remove the 2 screws holding the battery in. I just used a very small flat-head in one of the wings on the tri-wing screw and managed to undo them that way.
Cedric, to get to the trackpad you need to remove the battery and the battery is held in with tri-wing screws. I managed to use a tiny flat-head to remove the tri-wing screw but it was a bit tricky so I'd recommend getting a tri-wing screwdriver. That's the only special screwdriver you need to replace the trackpad. Everything else was small phillips head.