Remember that Yahoo offered pop3 access for free for a few years before yanking it. It takes alot of resources on the server side and without ads, there is no revenue for them. I doubt it will last long for free.
Just wanted to stop in--I talked to a Gmail person, and they said that they have no plans to charge for POP3 access. They're not promising that they'll won't have to charge sometime in the future, but I got the distinct sense that they'd like to keep POP3 access free, which seems pretty cool.