Just out of curiosity, is the stuffit expander not universal or something?
I downloaded a .sit file, and when I try to open it I get the "choose application" garbage. When I try to search for stuffit expander, it doesn't show up.
StuffIt Expander is current a PPC (Rosetta) app, but Universal Beta that expires on October 1, 2006 (release version should be out way before then) is available. It seems to work well (much faster than PPC version) but it seems to be having issues with password protected RAR files.