Here's how you do it: You download it with POP3 to the other gmail account.
In the account that needs all of it's mail forwarded, select this option under POP3/IMAP:
1. Status: POP is enabled for all mail that has arrived since xx/x/xx
(x) Enable POP for all mail (even mail that's already been downloaded)
In the account where you want the mail forwarded to, just download all your emails! Use you Google Account username for the username (without the @gmail.com domain), enter your password, and choose port 995, smtp server smtp.gmail.com amd Use SSL. Don't select "Leave a copy of retrieved message on the server" as gmail doesn't support it, and will give you an error.
There are other similar articles around the net but none have this solution.
I'm still waiting to be 100% sure it works as my account still says "checking mail" but it doesn't show anything yet in my new account.
Update as of May 20, 2009: It downloaded all the email from the old Google-Mail account but sent them all to spam. But hey, it worked! Now the last thing is export my contacts and import them into the new one.