Google encountered problems with Gmail Thursday morning for a "significant subset" of users using POP to get messages into Gmail.
The company posted a notice on its Apps Status Dashboard that was picked up by Techcrunch notifying Gmail users of "difficulties or delays receiving mail fetched via POP from external mail providers to Gmail." The problems began at some point this morning, and a more recent notice said that Google has fixed the problem for "a majority" of users.
Google said your e-mails that get imported from other accounts aren't lost; they're just not making it into your Gmail account in the usual fashion, and should be delivered when the problems ease. At some point mid-morning Gmail abruptly dumped a bunch of e-mails into my Gmail inbox that are usually imported relatively quickly from another account, some of which were over a day old.
Everything appears to be working on my end now, but let us know if you're having problems. After a few, it's been relatively quiet on the Apps Status Dashboard so far in 2010.