I did too, on the first Mac I bought: an iBook G4 in July 2004. I didn't get StuffIt on my iMac, though, which I bought in April 2005. I remember reading that Apple had stopped distributing it with new Macs, but I could be wrong. I do know that it doesn't actually come with OS X itself. A clean install of Tiger only installs Apple apps. Anyway, it's a free download, but I just wouldn't expect every user to automatically be able to open SIT files.