Snow Leopard removes support for Creator Codes, meaning when you double-click on a file, it is now opened based on the file extension or it’s “Open With…” setting, and is no longer opened based on the application which created it.

If you have files that are opening in the wrong application after upgrading to Snow Leopard, you can do a File->Get Info on them, and change the “Open With…” setting for each. To do this in bulk, try the freeware application SLopen.