I don't think it's a particularly BAD idea - short term it could TOTALLY mess them up - but LONG term, it's not too big a deal - especially when you consider that this idea is now being publicly advocated - this gives them a head start on constructing one and stockpiling refined products in the meantime.
I'd rather see a complete blockade - nothing goes in, nothing comes out - but we'd have to have some HUGE bargaining chips to convince Russia and China to go along with that.
....maybe that's why Bush has been pushing for us to start drilling in Alaska - so we can export that to China and Russia in place of Iranian oil. Sounds like a sizeable bargaining chip.
Even if this is the case, I STILL don't support drilling in Alaska!
I suppose we could also continually thwart their attemts and building a refinery through strategic missile strikes, but this would clearly be an act of war, and only make us more enemies in the region, and only build support for the Iranian government within the country.