RDC on here has done it as well.
You've really picked one of the worst controllers to build for a first project, I don't even build these anymore.
It really depends on how you're planning on doing it as to the different parts you'll end up needing. The way I preferred to build them, I use 2 of the older Six-Axis only controllers, one for the main board and it's Daughter board, then other for it's Daughter board and connector. The second one there is so you can keep the Triggers Analog. You may not want to go this route, since you're used to the PS3 it's firing is typically done on the R1 button, whereas on the 360 it's the RT, so you'd be swapping those lines around during the build so it was in more of a PS3 layout, thus no need for keeping the Triggers Analog. In that case you'll need a couple of NPN Transistors to do the Trigger presses on the 360 board.
You'll also need a newer version PS3 shell, because they have the Rumble motors and can hold the ones form the 360. If this isn't needed then one of the older Six-Axis only shells could be used.
The 360 controller you'll need will have to be the Wired CL version. Any new Wired Black controller you get now should have that version of board in it.
In the pinned topics of this forum are the links to the different 360 controller versions and info on them. For the PS3 controller versions, info on them can be found on these forums also in the PS3 Case / Hardware section. - http://forums.xbox-scene.com/index.php?showforum=341#
Good luck with the project, it's one of the more PITA things I've done (more than a few times) and I've decided to not go thru it anymore. I've decided the next one I attempt will be a wireless one, which has a whole new pile of challenges to figure out.