The site i used for the data sheet was -http://www.datasheetarchive.com/
There was even a small circuit diagram.
Wow, this just made my life easier.
If you can't find the data you need you can follow the traces on the board and work out what everything does.
I've been thinking of making a write up as I don't see any good links on byoac's site.
I did a mouse hack some time ago. I decided to clean it up as it was a mess. I think I screwed something up as it now kinda works

So I figured I'd start from scratch.
I think here's what I will do. I will take pics and write out how I am doing my current hack, if it works. And compare it to my old hack. My old hack had the 4 pin IR receivers, the new one will have the 3 pin receivers. I believe the only difference is the 4th pin is a ground or something like that. But same idea for both though when it comes to hacking. I will also explain how to use the datasheet if one is available (most likely one is) and how to do the traces yourself.
BTW, the other alternative is buy a prehacked mouse from oscarcontrols, only like $9 if I remember.
I get asked how to do a mouse hack often in chat. I'm suprised we don;t have anyhting linked in the interfaces section of the FAQ.