Unless U2 does some major promotion for the single, it doesn't really matter.
I have yet to see the new video U2-single commercial for the iPod. All I see is the Eminem iPod commercial. It may be possible that the U2 one will come out after Eminem's commercial runs its course for a few weeks. If and when the U2 commercial comes out, it may give the album a slight boost in sales.
But right now, U2 has done nothing to promote their singles in the U.S. after "Vertigo". The whole point of a single is to promote an album. But this isn't the 80's any more. Heck, even in the mid-90's U2 would still get signficant airplay and generate a hit song with a single. However, in 2005, the world has changed. Unless one is an R&B or rap artist, getting a song out there takes a lot of work and some luck - and often a TON of promotion. Being an older band and a rock band at that, U2 need to work harder at this promotion aspect than ever before.
There are exceptions to that last statement. Green Day seems to have this whole "comeback" thing working for them.. Some up and coming bands, like The Killers, will also generate a hit. But to have two or three hits from an album, U2 have to do more - and they haven't.
So unless U2 decide to do something to truly promote HTDAAB, and really push a future single, I see no reason to release anything else.