In the end, all politics are local. Tony Blair and leaders of other Coalition nations must turn public opinion in their favor before they face next have to face elections, which means that they have to consider not just the results but how they're perceived by voters. That makes it absolutely necessary for them to exhaust every opportunity at the UN and to make it absolutely unmistakably clear that the UNSC is at fault. The new resolution does exactly that.
The actual content of the resolution is lost in the press reports (which incorrectly describe it as a resolution to authorize war on Iraq) right now, but it lays the groundwork for a coordinated campaign to reclaim the moral high ground after the war, when the voters will hopefully be a bit more receptive. An active PR campaign will be absolutely necessary because those who opposed the war will pounce on every opportunity to declare it a failure. This also means that the effort to reconstruct Iraq must committed, sincere and scrupulously ethical (no sweetheart deals for old oil buddies) or cynical voters will buy into every oddball conspiracy theory.
I originally had a lot more to say on this topic, but work intruded long enough that pretty much everybody else beat me to it.
See the Opinion Journal, Porphyrogenitus or Arkat Kingtroll for more on the resolution itself and the situation at the UN. Steven Den Beste's latest has a ton of links and an analysis that contradicts his gloomy tone.
Andrew Sullivan emphasizes the importance of winning hearts and minds after the war and the importance of commitment to and good conduct in the reconstruction of Iraq.