According to the Google Enterprise Blog, Offline access is now available in Gmail Labs. To enable this feature:
- Sign in to Gmail and click ‘Settings’.
- Click the ‘Labs’ tab and select ‘Enable’ next to ‘Offline Gmail’.
- Click ‘Save Changes.’
- In the upper righthand corner of your account, next to your username, there will be a new ‘Offline’ link. Click this link to start the offline synchronization process.