Maxim Goldin talks about how to profile your Silverlight 4 applications in Visual Studio 2010.
This feature, however, is only partially integrated with Visual Studio UI, and you need to use command line tools to collect the data. After that, you can open the resulting .VSP file in Visual Studio as you normally do after other types of profiling, and enjoy rich visualization experience while investigating your performance problems.
Yes, we know it is not as cool as clicking VS buttons. We are working on a better experience. Yet I thought it is worth to emphasize that there is a way to use command line tools for customers that seek performance improvement in their Silverlight applications.