OK, first off, if I had to guess I would have Romney to win the Iowa caucuses tonight. From there I don't know.
As far as I'm concerned, the state of the race is that Mitt Romney can only be beaten if conservatives really rally around Rick Perry. That has not happened yet, which really helps Romney's chances. However, it could still happen in South Carolina. It's not at all difficult to imagine this happening -- it's perfectly reasonable to think that Bachmann, Gingrich, and Santorum could all be effectively or literally out of the race, leaving just Perry, who has enough rich Texas friends to keep running a campaign if he wants to. So Perry wins SC, gets 'comeback kid' status, gets endorsed by Gingrich and Haley Barbour and others, springboards to win Florida, and then we've got the 'long siege' Romney has been preparing for from the start.
I'm not putting good odds on that happening, but in my opinion that's the only way Romney could still lose the nomination, so that's the important thing to watch for. Thus, the important thing to watch for tonight is whether Perry finishes a reasonably strong fourth or maybe even third --or, an embarassing 6th that probably causes the rich Texas friends to pull the plug.
Oh, also keep an eye on whether Newt Gingrich -- who has done everything he can to ensure a devastating loss in Iowa tonight -- acts like he's genuinely pissed off at Romney, and will actively work to hurt him (which would include dropping out before SC).