Hi,
I put a little service called Spootnik (
http://www.spootnik.net) together which synchronizes Basecamp directly with OmniFocus.
I agree with Sheldon that OmniFocus is made for single individuals and Basecamp is a nice collaboration environment. Spootnik doesn't claim to make a 1:1 synchronization between them (also due reasons Sheldon mentioned before). But its target is to make the "Basecamp or OmniFocus" question superfluous. I'm convinced that they are good team-mates. I made some drawings here
http://www.spootnik.net/faces.html to visualize the possibilities a bit.
Spootnik is currently in private beta. If you like to test-drive it you can sign up here
http://www.spootnik.net/signup.html and join the private beta crew.
Best regards,
Lars