Hi RalfM,
Yes, you can use Spootnik without Basecamp just to synchronize your OmniFocus devices. But that way it's only a WebDAV provider like MobileMe. Thus, you're not able to synchronize a subset of tasks.
You can synchronize a subset of tasks if you maintain different OmniFocus databases (e.g. everyone has his own database) and Basecamp is in the middle. Spootnik keeps everyone up to date (see
http://www.spootnik.net/faces.html)
- Lars