As someone who's overseeing a rather huge implementation of SharePoint 2007 - and as a former user of Confluence - I have to say that SharePoint does pretty much what Confluence does (wiki, document sharing and versioning, calendaring) except that SharePoint has far more overhead. SP setup and deployment is highly complex, and it's very difficult for most users to fathom, requiring hours of training. SharePoint does have some unique capabilities (e.g. Office Suite integration - as long as you're on Windows using IE - and only half of my audience meet that requirement), but overall Confluence wins hands down IMHO.