We'll see how Santorum does in the next few primaries. He'll probably do well in Alabama and Mississippi. With Kansas, I have no idea. In Illinois, which would probably be a good state for Romney anyway, Santorum faces the same problem as Ohio, where by default he can't win some of the delegates there due to paperwork issues.
Romney will be the nominee; it's a matter now of how long Santorum can drag this out. The only outside chance Santorum has is keeping Romney from getting to 1144 delegates, and having a brokered convention (which probably won't happen).
Gingrich and Paul will stay in, but are both becoming less and less of a factor. Gingrich has been out for a while, with everyone but him knowing it. He'll continue to chip a few points off Santorum's totals in some states. Paul really needed a better performance on Super Tuesday to build/keep any momentum. If Maine had turned out better for him, or if he'd picked up even one state earlier this week, he'd still be a factor. He wouldn't have a shot at the nomination, but the upcoming primary season would have a lot less certainty.
If Santorum does well in a few upcoming states, I could see Romney not sealing the deal until May. If Santorum doesn't do well in the next few contests, Republicans will accept Romney sooner.