Google notes that "the 'go Google' and 'Google Apps Migration for Microsoft Outlook' tools have helped unshackle thousands of organizations from their previous technology solutions. Our customers have migrated more than 2 billion email messages to Google Apps, and in the past 2 months alone, more than 14 million calendar events and 6 million contacts (not counting domain-wide address lists) were migrated to Google Apps from Microsoft Exchange and Lotus Notes."
Here's a summary of data migration options:
- Google Apps Migration for Microsoft Exchange
- Google Apps Migration for Microsoft Outlook
- Google Apps Migration for Lotus Notes
- IMAP mail migration tool
- Google Apps Email Migration API
- Google Documents List Data API