I think what you may be referring to is Snow Leopard's ditching of type and creator codes for Uniform Type Identifiers, which there's some detail about here and here. It doesn't do exactly what you're asking about here, although it certainly influences what application ends up getting started by Launch Services to open a file.
That said, there are some 3rd party utilities that might do something similar to what you're looking for (though not exactly what you're requesting) like Magic Launch or LaunchCodes. There might be some other ways to hack/modify the Launch Services database yourself w/o a piece of shareware, but I'm not aware of them.
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To do exactly what you're asking for, I think what you'd actually have to do is modify the Info.plist within each of the applications you don't want on the list, and then rebuild the Launch Services database.