Since then, I've set up Apple Mail to hit Gmail via IMAP. If you've already been using POP3 access, you might want to disable Mail from copying all the messages in the IMAP server to your account -- you can do that in Preferences ->
Anyway, the main thing I wanted IMAP for was to be able to copy up pre-Gmail messages from Mail to Gmail -- the newly found headroom in Gmail storage gives me more than enough to get at least all of this century in there. But Mail doesn't seem to like being told to copy thousands of messages all at once -- it's crashed on me a few times with big batches. So I'm plodding along a few hundred messages at a time, copying them to "All Mail". Should be done later today.
Luckily Mail has smart folders now, so I just have one set up to show me all the messages that are not in Gmail and have a received date before July 2004 (which happens to be when I started using Gmail).
The cool thing is that when you copy messages into "All Mail", all your regular filters get applied, so I'm getting labeling on all these old messages too. Before I'm done, I'll do a search for "before:2004/07/14" and apply the label "older-imports" so that I have a single place to find all my old mail in the future.