Long title, I know. Here's how I did it...

sudo apt-get install fetchmail postfix
cd
nano -w .fetchmailrc

Add the following to the file (replacing everything in CAPS appropriately):

poll pop.gmail.com with proto POP3 and options no dns
user 'YOUR_GMAIL_ADDRESS' there with password 'YOUR_PASSWORD' is \
'YOUR_UBUNTU_USERNAME' here options ssl

Now:

chmod 710 .fetchmailrc
sudo nano -w /etc/crontab

Add the following to your crontab file:

10 7 * * 1 YOUR_UBUNTU_USERNAME fetchmail -k

This will run fetchmail every Sunday and grab your email. It seems either fetchmail or gmail limits each connect to something around 400-500 messages, so to get started, you can run "fetchmail -k" a few dozen times to download all your mail, then let crontab do the rest on a weekly basis. Your mail is downloaded to a single mbox file in /var/spool/mail/YOUR_UBUNTU_USERNAME. Being a single file makes it nice for backing up, moving, etc. I hear Thunderbird can easily read the file, and I think Apple Mail.app can as well. Just makes you feel good having all your mail backed up, now doesn't it?