In what can be called an "oops" moment, Apple this week provided security update 2012-001 to Snow Leopard users to keep the older version of OS X in step with the security updates that were part of the OS X Lion 10.7.3 update.
Courtesy of Apple
Apple kept the two versions of OS X in step a bit too well. Many users are reporting that after applying the Snow Leopard security update, most, if not all, of their older applications that rely on Rosetta to run have stopped working. Office for Mac 2004, Quicken, and just about any other application that was created using the older Carbon APIs that Apple supplied for PowerPC-based Macs have stopped working.
The purpose of Rosetta was to serve as a bridge between older PowerPC-based apps and newer Intel-based Macs. Rosetta isn't installed or supported in OS X Lion, but obviously Apple didn't intend for Rosetta to stop working under Snow Leopard. The Snow Leopard update must have included some bit of the underlying security code base for Lion.
Apple will no doubt address the issue. In the meantime, hold off on updating your copy of Snow Leopard until Apple fixes the problem, or (if you're a Quicken user) until the tax season is over, whichever comes first.