We recently discovered that Enterprise SSO service fails to start if you install .NET 4.0 Beta bits. This causes a downstream failure on Biztalk services failing to start as the dependant ESSO fails. We are still investigating the possible fix and I will update this post once we have a resolution. Please do no install .NET 4.0 on existing Biztalk installations. The issue applies to all versions of Biztalk and SSO.