[Lizard]

Deleting OmniFocus.ofocus from ~/Library/Application Support/OmniFocus should be enough to reset everything.

To get an extra-clean start, you could do the following:
1) Make sure your macbook has all your current data, no changes on your iPhone waiting to be synced over.
2) Move aside or delete the OmniFocus.ofocus file on your iDisk.
3) Reset the database on your phone
4) Move aside or delete the OmniFocus.ofocus file on your iMac.
5) Sync from your MacBook OmniFocus to MobileMe
6) Sync your iPhone OmniFocus to MobileMe
7) Sync your iMac OmniFocus to MobileMe