Since we're linking to this thread from the FAQ, I should mention that is now a lot easier. Rather than needing to find the database file yourself, we've added options to the preferences pane to move it for you.
Open the General pane of OmniFocus' preferences on the source machine - the one you want to move your database 'from'.
Press the "Move to a New Location..." button in the Database section of the window.
Select the location you'd like to move your database to - some folks put their database on a USB drive so they can take it with them to work, for example. Other folks put the database on their iDisk or some other network location that both machines can see.
(Keep in mind that writing to a network location is much slower than writing to a drive attached to your machine directly. Your mileage may vary.)
In any case, press the Move button, and OmniFocus will move your database to the new location.
Once the file is moved, OmniFocus on the source machine will know to look in the new location. For the destination machine - the one you want to move your database 'to', the procedure is a little different.
Note: the following procedure will replace the existing database on your destination machine. You're essentially telling OmniFocus 'hey, I want to use this database instead.' Proceed with caution. ;-)
Open the General preferences pane again, but this time, press the "Select a Different Database..." button.
Locate and select your OmniFocus database in the sheet that slides out, then press the "Open" button.
OmniFocus on your destination machine will now look for your database in the location you chose in the previous step.
Hope this helps!