I this issue too- about a week ago.. Apps would download, but they would take forever (about 5 minutes). I haven't tried since then, so I can't give input as to it is still doing it (sory), but I can confirm that I didn't do any updates or upgrades to my phone..
I DID notice that I had this issue after I cleared the cache on my market app though...
Did you by any chance do anything like that?