With the release of 2010 on 12 April, MSDN Premium subscriptions are moved to higher level products. This is all outlined in the VS2010 Roadmap, and the details of what is involved in each subscription is outlined in the MSDN Subscriptions Matrix. So if you have VSTS Development Edition, you are upgraded to VS2010 Ultimate.
This was going to expire on 21 March, however is now aligned with the VS2010 launch date, so is open until 12 April. This promotional offer is a great way to step up the version of VS that you are using, at a considerable cost saving.