The following did work for me and my 16GB+ mails:
- Copy the following files (e.g. from your Snow Leopard backup) prior to first opening ical:
~/Library/Keychains/ (folder "Keychains")
~/Library/Calendars (folders "Calendars")
~/Library/Preferences/com.apple.iCal.* (ical preferences files, com.apple.iCal.helper.plist, com.apple.iCal.plist)
~/Library/Preferences/iCalExternalSync.plist
- then open iCal in Lion...import/update will follow
If you have opened iCal prior to the above steps, these files have already been created, so you have to replace them with your backups from Snow Leopard described above
If you have set up calendar accounts in system preferences "Mail, Contacts & Calendars" you might have to delete them before applying the above steps
P.S.
I'm not quite sure if the following steps are necessary (or did help me) for iCal import because MobileMe sync works now different in Lion (there isn't an option for syncing calendars anymore within System Preferences—>MobileMe)...
I did a MobileMe sync in Snow Leopard (within System Preferences—>MobileMe) prior to the clean install of Lion.
- Calendars
- Mail Accounts
- Mail rules, Signatures, and Smart Mailboxes