Rob points to a post in which Dana Epp comments on a sneak preview of Visual Studio Team System. Dana muses,

If I have one beef with Team System, it is actually something Microsoft thinks is its biggest strength. It is so tightly integrated that it will not be easy to integrate external source control and bug management tools directly. This could be bad. A development environment who may have shelled out tens of thousands of dollars for a redundant Perforce solution isn't going to look to kindly on the thoughts that you need to now use Microsoft's new and untested source control management system. (Lets not get into the old discussion about how much VSS sucks... everyone knows it... including Microsoft). For me I use Subversion with TortoiseSVN (you can read about my experiences setting it up here) and am now looking at moving my bug reporting tool from BugZilla to something like FogBugz. The last thing I want to do is have to alter our development process again... I am just finishing doing that. I want my existing repo to 'just work'. I'm afraid thats not possible.

I haven't been involved in planning VSTS but I have heard over and over again that we're building a platform, not just a suite of tools. It's unlikely that Team System will be completely optimized for third-party integration in v1 but I can tell you that Microsoft continues to invest heavily in provider-agnostic source control services for Visual Studio. No, that doesn't mean that you'll be able to plug Perforce into Team Foundation, but you can certainly plug your source control provider and a third-party work item tracking application into Visual Studio to achieve the same result.