Today's Scotsman reports here that "government spokesmen" have indicated that Tony Blair will not back any strike on Iran "regardless of whether the move wins the backing of the international community". I think the important point is in the last line, as presumably "international community" means the U.N. So thats seems a pretty definite "No".
Furthermore, the rationale given (the emphasis on a lack of British military resources available for the strike) is interesting as well. It has become clear that the U.S. military is not the only force to be overstretched by its Iraq commitments. Britain is clearly looking past the current situation to what they might face in the aftermath of an American bombing campaign against Iranian nuclear facilities.
This is actually good news because, Seymour Hersh aside, an actual military assault looks less likely in the short term. For one thing the States would be acting alone (or perhaps with Israeli assistance), and would bare the consequences pretty much alone, through (at least) increased casualties in Iraq via clashes with with Shia militants in areas of the country that have been, until now, relatively pacified.
For another, I don't see how the timing would work out. The Bush administration has always conducted its military campaigns in Iraq and Afghanistan with both eyes on the domestic political calendar. For example, Fallujah was allowed to fester until mid-November 2004, after Bush was safely re-elected, so the news cycle wouldn't be filled with accounts of dead soldiers and bombed out hospitals during the campaign itself. Since mid-terms are coming up this November, and the GOP is looking vulnerable, a strike would not make political sense before that time unless it could be absolutely guaranteed to be a success, which is of course impossible. Otherwise the big election story is kids coming home in body-bags.
What I can see happening is an attempt at replaying 2002, when Bush and the GOP used a lot of chest-thumping to paint the Dems as weak on National Security in the run-up to the Iraq War, and gained in both the House and Senate. Attempts will be made to convince the U.S. electorate that the organs of the Federal Government must get behind a possible military strike in Iran (date to be announced sometime after the elections), and that, once again, any dissent smacks of disloyalty.
Whether that works this time around, and what happens after November 2006, I will not venture a guess.