I'm more surprised to learn that it wasn't supported from the start. VB.Net creates .Net bytecode just like C# and can already be used to create Silverlight apps. They are usually kept in parity. I guess this is kinda parity, though, since the phone isn't even out yet.
FWIW, we even stopped trying to teach beginning programmers in VB. It is simply too inconsistent in its syntax. It was never intended to be true OO--it's just kind of tacked on and very convoluted, especially for a beginner. BUT, I know a lot of potential WP7 app devs still use it religiously, so it's good to see support added.