Seems the latest round of patches released by Apple this week does a number on the 64-bit side of things. They're not sure yet whether or not all 64-bit apps are affected, but they know that Mathmatica's not running on systems with the patch applied.

"In particular, the problem means that Mathematica 5.2, released in July, will not run on a G5 computer with Mac OS X and Apple's latest security update, according to the e-mail. Apple is aware of the issue and plans to fix its security update, Ben Wilson, a Wolfram spokesman, said in an interview on Wednesday."