From the
FAQ (sticky):
Q.Can I synchronize my data between multiple computers?
A.We're working on synchronization now that we've finished the basic single-computer functionality for our 1.0 release.
For the time being, you can place your OmniFocus database on your iDisk and access it from your various machines. But I should note that you're definitely heading out towards the land of bleeding-edge beta testing. We want feedback from folks, but we don't want folks to set this up casually and then be upset if something they didn't expect happens. Proceed with our blessing, but also with caution. We recommend you set the backup preference to 'twice a day and on quit' on all your machines. ;-)
On one of your machines, open up OmniFocus' "General" preferences pane and use the "Move" button to get your database on your iDisk.
On your other machines, open up those same preferences and use the "Select a Different Database" button to select your database on your iDisk.
You should now be all set.