I’ve been moving to GMail lately. The original reason was being out of the office fairly often these days, and a good webmail is very convenient. And GMail’s search-based features rock!
My mail archives are stored in my macosx’s Mail.App, and in recent versions Apple has moved away from the mbox format for storage, so the move is a bit more involved that I’d have liked it to be.
Here’s my recipe:
Start with the cosmicsoft emlx to mbox converter. I’d have preferred a command-line thing, tried Jamie Zawinski‘s emlx converter but the output format didn’t seem to be correct.
Next, add a marker in the subject lines, and setup a filter in GMail so that messages are marked with the correct GMail label. You don’t want thousands of old messages in your inbox.
sed s"/^Subject: /Subject: (whatevername-g) /" mbox-with-markers
Finally, use Mark Lyon’s GMail Loader to forward the contents of the mbox to your GMail account.
It takes a while…but it’ll be useful to have everything there.
Is it safe? I trust Google more than myself about backups, for sure. And there’s no privacy in unencrypted email anyway, so I don’t care much about what they do with the data.
bertrand's brain grep 
only saw this now (thanks to Patrick), but what’s wrong with Thunderbird as mail app?
Nothing wrong with Thunderbird, of course! It’s just that when I moved to MacOSX I tried to use the standard tools there, and Mail.App was good enough for me.