Beta 3 for Team Foundation will be released in September along with Visual Studio 2005 Release Candidate 1.  One of the most significant points with beta 3 is that it will have a go-live license that includes data migration to RTM.  The details are in Soma's blog post for today: Visual Studio 2005 update.

Here's a copy of the part that's TFS-specific.

Simultaneously with Visual Studio 2005 RC1 [in September], we will also release Beta 3 of Visual Studio 2005 Team Foundation Server (TFS). TFS is Microsoft’s server-based product for team collaboration and is part of the Visual Studio 2005 wave of products. 

TFS Beta 3 will include a Go Live license along with technical support for Premier customers, enabling organizations to begin deployment of their Team System collaboration tools immediately.  The Go Live license will also enable us to solicit an additional round of feedback from customers prior to shipping.  We have received fantastic feedback from customers, partners, and our own internal use so far.  Over the next several months, I want to encourage you to exercise TFS under your real-world conditions and send us feedback via the MSDN Product Feedback Center.

At Visual Studio 2005 launch, we will continue supporting TFS Beta 3 with the rest of Visual Studio 2005.    Further, all data within TFS Beta 3 will migrate seamlessly and in-place to the final version of TFS.  We will ship TFS in the first quarter of 2006.