As you may know, Team System is not just for developers, but for all members of a software development team - testers, project managers, and architects, as well as developers (and, if you look at XNA, artists, sound engineers, modelers...the list goes on).
Not all of those folks want to install/use Visual Studio, of course. So we're providing a couple of alternatives. The main "out of the box" choice is simplly called 'Team Explorer' (we also call this 'TFC' internally - Team Foundation Client). If installed on a machine with VS2005, it integrates with VS and provides a great integrated experience for developers.
But, you can also install it on any old machine (hardware requirements aside, of course), and still get a pretty big chunk of the Team Foundation functionality (using only a Client Access License in terms of 'deployment cost'). For example, you get the Source Control Explorer and command line client for version control, all the Work Item Tracking features (at least, all the ones I've ever tried to use - I'm not a WIT expert), etc. So, your PMs can review and manipulate work items(in the TFC client, or in Project and Excel, for example). Your branch admin can perform a merge. Your testers can get access to the bug database on a box sitting in the test lab. I'm sure there are other scenarios I haven't thought of.
That leads to my question: How do you think your organization will use the TFC? Only in VS? Will you only install it on PM machines to get access to Office Integration, or would they likely use the client itself as well?
On a related note, what number (or percent) of your team do you think would use "only" TFC, as opposed to installing it to integrate with one of the VS SKUs (VSPro, VSTS, VSTA, etc.).