For me...Win2K MSIE6. I can open Acrobat...then go to Edit -> Preferences -> Options. In there, there is a spot that says "Web Browser Options". Under there, there is a check box that says "Display PDF in browser", if I uncheck that, then it asks me to open or save it.
I do not know anything about Macs though, so I cannot say if this'll work or not...can't hurt to try though.
HTH
Mike