As a follow up to this post, I gathered all the GOP primary head-to-head matchups since Rick Perry entered the race that I could find, and then selected just the ones featuring the current Not Romney at the time of the poll.
There's not a lot of data points, but it seems to confirm the notion that roughly half the Republican primary electorate is extremely eager for a candidate other than Romney and that if they can ever find a candidate they are willing to settle on, Romney's in serious trouble. Of course, as we've seen throughout the primary so far, that's easier said than done, but if Romney does win the nomination, he'll do it because of the weakness of his Republican opposition—not because of the strength of his campaign.