I had a couple of Microsofties come in to work to present to us about Virtual Earth. They talked a lot about VE's Silverlight integration, but when asked they admitted that only about 35% of desktop users had Silverlight installed. Even if that is not a high estimate, it's pathetic.
Even if you only care about Windows users, Silverlight is not a suitable technology to roll out to end-users. Flash 9+ has something like 98% market penetration.