I want to open a few select files with a non-default application (now that the creation code is not used.)...If I work on the file and make/save any changes, it reverts to the default app.
Aha! Okay, I don't think any of us were getting that in the initial posts. You are correct. Snow Leopard no longer uses Type and Creator codes
by default, but they are still there. Various things are keeping you from doing what you want.
1) Newer programs that do not use Type and Creator codes at all still need a way to tell the OS what files belong to them. Since at least back as far as Leopard (likely earlier), the system now uses Uniform Type Identifiers. As far as keeping track of what files belong to what applications, they serve the same purpose as Type and Creator codes. So when you save changes, it will indeed revert back to the default app since the file once again gets saved with that program's UTI data.
2) Some programs use both UTI and Type and Creator codes. Usually older apps that have come from PPC code. I would guess that would be either because the developer doesn't want to waste time removing the code that adds the Type and Creator codes to the files, or more likely, to make sure files passed to other users still on older Macs that don't use UTI won't have trouble opening them.
3) I use some much older apps which have still not been rewritten at all for Intel Macs and are still all PPC code. They still write Type and Creator codes.