Webmail users: You'll need to move your calendar, contacts, and briefcase items. These can be exported & imported and only take a few minutes. We have instructions and videos to show you how. See Migration Instructions [1] You'll also need to re-share any calendars that were shared. If you used filters, tasks or documents, you'll have to manually move these. We'll provide suggestions for making this easier.
Outlook users: Your ITC [2] will help you get Outlook switched over to use Google Apps Sync [3] for Outlook. As part of that process, they can import your calendar, contacts, local mail, notes, journal entries, and tasks. You'll need to re-share any calendars that were shared and recreate your filters.
POP/IMAP users: If you use some other mail client, you'll have to change your POP or IMAP settings.
Links:
[1] http://www.humboldt.edu/its/google-move-zimbra
[2] http://www.humboldt.edu/its/glossary/5#term233
[3] http://www.humboldt.edu/its/glossary/5#term43
[4] http://www.humboldt.edu/its/google-macmail
[5] http://www.humboldt.edu/its/google-imap
[6] http://www.humboldt.edu/its/google-pop