There are reports speculating that Romney and Paul have entered into some kind of agreement, perhaps a role of some kind for Paul if Romney wins the nomination or presidency.
Apparently with all of the mud slinging it is noticeable that these two seem to go out of their way not to discredit the other.

For instance when Romney painted his opponents as Washington insiders, he expressly excluded Paul from his castigation even though the congressman has amassed much more time as a Washington politician than Santorum or Gingrich. Moreover, Paul's sudden and puzzling labeling of Santorum as a "fake" is peculiar. Romney and Paul appear to be an odd partnership, but politics have created far stranger bedfellows. Moreover, at this time it would appear to be in Romney's benefit to have Paul stick around in the race as his followers may well be more inclined to support Santorum over Romney.

I felt that Gingrich had had some talks with Perry, possibly providing some conditional concessions, to get Perry, who was gung ho about South Carolina, to drop out of the race and endorse Gingrich. It was immediately after Perry's withdrawal that Gingrich experienced his biggest surge, passing Romney in the polling.