Looks like Gmail is about to officially offer POP access, according to this message in their support database. This function has already been enabled by lots of third party tools, but has been expected from Google themselves for quite a while. It doesn't appear to be actually be enabled yet—not for me, anyway; a few people are reported to have gotten it working—but it looks like it is just around the corner.
Text for their configuration instructions is incomplete, make sure to use port 995 for pop.gmail.com and to enable SSL. One current drawback is that mail sent from the web client appears in the Inbox.
This is good news for mobile devices, as even the most basic usually have a POP client of some kind, even if they can't run Gmail's web-based client. And that means using Gmail as your primary email account is going to be a lot more viable. (Thanks, Vic and Animaniac!)
Support Page [Gmail.Google]
Update: It's official. They say they're intro'ing it in "phases."
Why don t I have POP access? [Gmail.Google]