Hmm. I guess it is after all sort of the 11th. I got that, too. I clicked D&U, dialog box dismissed, then it just went back to showing I was up to date. Clicked Check For Update button again, got the usual "your iPhone software (4.2.1) is up to date. Did it again, got the 4.3 update dialog again, but again it wouldn't updated.
Excuse me, Mr. Croupier, could I just pick up my chips off that there black 11, please?
Update: yeah, me too: downloading now, slow as molasses