I believe Google Apps has done a fantastic job of this. Each document can have different people who are invited to both view and edit the document. As well, you can provide the visitor with rights to invite more people. Above all, it has the entire trail of changes by every user at every moment the change was made. You can track any change directly back to the person editing the file. Best of all, you can set up Google Apps to only authenticate on your domain and you can import any type of Office document into the system.

I've had a look at Bluelock http://bluelock.com/ and what they're doing with VMWare's cloud technology may be the difference you're looking for. VMware is extending a virtual cloud to any datacenter running VMware - that's a pretty fascinating approach that could put the big boys on their heels! http://vmware.com/cloud