This might be true for the "American street" but it just isn't going to happen among Europeans (or activist Americans). Best case, you'll see a swing of ten percent or so - possibly significant but far from decisive. Even ordinary Europeans "have their blood up" now and are likely to resist the notion that the war was justified. Worse yet, accepting the war, even after the fact, clashes with fundamental pillars of the European world view ("War bad, UN good"). Finally, the fact that every picture of happy liberated Iraqi's will be matched in the press by a picture of dead or injured children means the upside for the pro-war view is pretty limited. That's not to say that opinions can't be changed over time, but it will take years of patient and careful work rather than one big PR "shock and awe" assault.
Tony Blair is frustrated, disappointed and angry at the UN Security Council and especially at France. Despite that frustration, he is deeply committed to the concept of the international law and institutions and is not ready to give up the UN as a whole. He will likely try to reform the UN, but he won't throw it overboard. Blair might be persuaded to switch horses if France continues to wield the UN as a political instrument and an acceptable alternative gains some kind of political momentum, but that's about as far as he (or British voters) will go.
This non-starter just compounds the two other bad assumptions into something even more disastrously off-base. The concept of the UN and other international institutions is fundamental to European views of a peaceful world, and they're deeply suspicions of an activist America especially one that's led by the idiot cowboy Bush. Nothing will galvanize European and world opinion against the US faster than a perceived attempt by the superpower to destroy the UN, and no politician in Europe could stand against that. Not that even the most stalwart ally would even try (see bad assumption 2).
Bush can probably get away with just ignoring the UN, but Blair simply cannot. France is pushing the issue at the UN now to force Blair to choose between backing the US (which would be political suicide) or backing the UN (which would damage US-UK relations). Chirac may have overplayed his hand by being so trigger happy with the veto threat -- his belligerence allows Blair to treat the whole question as a dispute with Chirac rather than a dispute with the UN. The reprieve is temporary, though -- Chirac will tune his message and if Russia and China join in it will get harder for Blair to avoid the issue.
Update: In the comments on the original thread, Rob Robertson points to another danger. France is reported to be mediating an attempt to get Iraq to "surrender" to the UN, which would also put Blair in a tough position.