I've been playing with Visual Studio 2010 Beta 2 and .Net 4.0, building code, running tests, playing with the IDE, etc. The first issue that I've come across with my existing codebase is that the .Net 2.0 hosting APIs (such as CorBindToRuntimeEx)are now deprecated and there's a whole new way of hosting the CLR.
We've been quite successful in hosting the CLR from within our C++ servers, either to provide servers that support a mix of managed/unmanaged plugins as a pluggable high performance windows application server or to provide network protocol support in C++ (such as ENet) with 'business logic' being written in managed code. The .Net 2.0 hosting API works OK but is not without some annoyances. Over the next couple of weeks I hope to take a look at the new hosting interface and report here on my findings. With any luck the server framework will support the new hosting interfaces in release 6.2 which currently has no scheduled release date but which I expect will appear early in 2010.