I've been using FolderShare to synchronize my OmniFocus document between my home and work Macs as well as my MacBook. Although it now has a big ass Microsoft Logo stamped on it and doesn't offer a universal version of the syncing client, it works really well. You have to absolutely make sure you're not opening the same (synced) document from multiple computers at once, though (I'm using Apple's Remote Desktop in conjunction with AppleScript to quit any copies of OmniFocus on my other machines, if necessary).

For what it's worth, FolderShare doesn't actually store your files on its servers. It simply interconnects your machines to exchange files in a peer-to-peer fashion.

For the record, the default OmniFocus document is stored in ~/Library/Application Support/OmniFocus/OmniFocus.ofocus and yes, double-clicking an OmniFocus document in a different location works nicely (for example with the regularly backed up copies in ~/Documents/OmniFocus Backups/).

Best,
Patrick