What is surprising is the role that he is expected to fill in Microsoft. He is going to work full time on SubSonic, an Open Source project.
Why is this surprising? Because to date, I haven't heard of any other cases where Microsoft have paid for developers to work full time on OSS that didn't came directly from Microsoft. This is a fairly common model in the OSS world, but this is the first time that Microsoft has made such a move.
Very welcome move from Microsoft, and congratulations to Rob.