I'll answer your question, then follow up with one of my own - because I'm one step ahead of you, but with a follow-up problem...
Navigate to your yourusername/Library/Mail directory and copy that entire directory to the same directory on the new machine.
(I went this route when moving settings from my old iBook to the new iMac... as I didn't want to copy absolutely everything over...)
Open mail on the new machine and you'll have all your current mailboxes, messages and server settings all set-up on the new system.
Worked for me.... so far... except my problem now:
I have to manually enter all the account passwords each time that I restart Mail. I use the checkboxes for 'remember in keychain' but my passwords are not being remembered. (They'll be remembered as long as mail is running, but not after)
Someone please help! I'm going nuts having to retype DJEY4557, JKHU3443, QQBD9008 and apple each time I enter Mail. (not my real passwords....)
I've done the standard 'repair permissions' and used Cocktail to run all the Chrons. This is my brand-spankin' new iMac G5 and this is my only gripe so far.