I have tried to use VB under Mono, but I haven't got it working yet - there seems to be some issue with the VB compiler, I may try again. But I'm dubious still about if Microsoft will let people use it for several years, and then pull a patent out of the bag and make us pay loads of money to continue using it - I doubt their motives basically.
Actually we're on pretty safe ground. Microsoft released the "API" on how the CLR runs as an open specification - they actually did several rounds of the universities plugging the concept of a fully open source implementation of .Net. Their motives are that they can provide a better .net framework then any open source one, and can be driven to providing better by the competition.
I believe a quick google of mono and microsoft will provide some more info on the commercial agreements between Mono and Microsoft that have been put in place because there was a bit of patent infringement, although I think the FSF had some concerns that this was evidence against Microsoft's own Community Promise ( http://www.microsoft.com/interop/cp/default.mspx ).
Microsoft actually have a history of wanting people to use their development environment and language. At the end of the day, the best release of .Net will probably always be the Microsoft one running on Windows. Even if they are an evil corporation out to eat our souls, It dosn't stop the language being a bloody good one.