Macbooks and Mac Mini's with the x3100 (and 950?) haven't been able to run Diablo II and play on Battle.net for about a year and a half now.
There is a beta patch that allows it to work, but you can't play online.
Supposedly 1.13 will fix this, but who knows when that is coming out because they stop working on it to fix Warcraft III hack issues.
All I can say is, this kind of stuff doesn't happen in Windows. Apple basically removed 256 color modes from their graphics driver for this GPU, breaking SC and Diablo II. SC was fixed already. In Windows, you could just wait for a new driver to come out and then you'd be back up and running.
The problem in this case is two fold. Apple breaks it and hasn't fixed it since with the many updates to Leopard. Blizzard will fix it, but it has taken them so long to do.