Elaborating a bit more on my original post:
If you are syncing between multiple copies of OmniFocus, you'll want to always do the archiving on the same computer, as the archive is not currently synced. Also, only the Mac application can do archiving.
Retrieving information from the archive is easily done with File->Open Archive. You'll get another window with the archive database, and you can drag and drop in either direction.
The problem with the archive support from my point of view is that it is exclusively date-based. If you have a completed project that you need to keep in your active database, that restricts OmniFocus to archiving only material completed prior to that project. You can archive projects by hand by dragging them to the archive and not doing any date-based archiving, but that loses one of the best features of the archive, which is its ability to harvest completed actions out of incomplete projects. If you've got single action lists full of repeating actions, the archive command can keep them relatively trim.