Besides bug fixes and some added functionality, Update 1 also adds support for Exchange 2010 SP1. Because Exchange 2010 SP1 doesn’t support the get-antispamupdates cmdlet (see this post), implementing Exchange 2010 SP1 on servers in the Mail protection role (with Exchange – Edge server role – as well as ForeFront Protection for Exchange) leads to issues.
Update 1 fixes this issue. To make things confusing, the ForeFront team calls these cumulative updates Software Update or Update; what’s wrong Rollup? Be advised that the ForeFront Update page doesn’t mention the Update (yet), nor is the related knowledge base article published (kb2288910).
You can download ForeFront TMG SP1 Update 1 here. Note that currently only English is available, other languages are said to be made available soon.