From a guy who would be able to determine such things, NRO's Byron York is seeing some signs:

We are beginning to see signs that Mitt Romney might be preparing to pull out of the Republican race.

Everyone is looking for tea leaves from the campaign, and we'll know more about the governor's state of mind when we hear from him at CPAC today. But in the last 24 hours, I have noticed a dramatic drop-off in the number of emails I have received from Team Romney. This is no small thing. Every reporter will tell you they receive reams of emails from the Romney campaign. But the last one I got was at 1:47 p.m. yesterday, advising me of Romney's campaign schedule in Maryland. Before that, I last received on at 8:37 a.m., advising me of Romney's CPAC speech.

Normally in that time period, I would have received lots of emails telling me, among other things, that John McCain's Straight Talk Express had taken a detour. For example, I would have expected to receive one about McCain's "calm down" remark. But nothing from the Romney camp. I wondered whether the campaign is dialing back its aggressiveness in preparation for a Romney withdrawal.

A few moments ago, I spoke to someone in the Romney camp. Would I be crazy to read that into the email traffic? "You would not be crazy to read that into it," he said. "There have been a lot of discussions going on about whether there is a path to victory, and not wanting to look destructive at what might be the end. You are reading the right thing into it."

Here's the thing about Romney's chances: It's next to impossible for him to win the delegate race. To get to the needed 1,191 delegates to win the nomination, Romney would have to win almost every delegate from here on out. But there are only three winner-take-out primaries left: Virginia and D.C. on Feb. 12 and Vermont on March 4. And even then, assuming Romney won all three, that would only net him 99 delegates.

Romney's folks have put out the word that the campaign's hopes rest on Ohio and Texas, a total of 222 delegates. But again, both of those allot delegates proportionally.

UPDATE: Mark Halperin is a bit more definitive.