Iowa is over already. He's won there, unless everything in the world changes, in which case he's lost in some other dramatic, unexpected way.
That leaves Colorado and New Mexico. He has a very solid lead in New Mexico and is doing great in Colorado.
After that Nevada, Virginia, Ohio, Florida, etc. are all icing on the cake. If he gets them great, if he doesn't, no problem at all.
Here is another way to look at it. Assuming for a second that unless something crazy happens, Obama will almost certainly win Iowa and New Mexico. That means he only needs one of these states: Colorado, Ohio, Florida or Virginia.
There are other combinations, too, of course. But the point is if you look at it this way, this looks like almost too steep a hill for McCain to climb even if things were going well. And they definitely aren't.
This thing is teetering on the edge of being over.
Update -- It turns out Karl Rove agrees with me. Check the bottom of this post out.