As part of the installation process, Snow Leopard takes unprompted ("automagic!") action to cordon off application programs that have been deemed incompatible with the new OS release.
There are two categories of offending software: Incompatible and Restricted. "Incompatible" apps are moved to an "Incompatible Software" folder, while "Restricted" programs stay where they are but are prevented from launching.
Apple has details in a Knowledge Base article. I've excerpted the lists below. More problem apps are sure to follow.
- Parallels Desktop 2.5 and below
- AT&Tís Laptop Connect Card (1.0.4, 1.0.5, and 1.10.0)
- Norton AntiVirus 11.0
- launch2net 2.13.0 (for some 3G USB dongles)
- Aperture 2.1.1 and below
- Keynote 2.0.2 and below
- Parallels Desktop 3.0
- EyeTV 3.0.0 to 3.1.0
- Ratatouille 1.1