When attempting to use a Silverlight application in the 64-bit version of IE you will get this message. “Microsoft Silverlight cannot be used in browsers running in 64 bit mode.”
At first I was disappointed because I would like to see more of the industry move to 64 bit. Heck they are selling it at Best Buy now. However does Silverlight really need 64 bit? 64 bit gives you more throughput on the processor and more access to memory. But it is defiantly not typical for Silverlight applications to need that much memory and/or do highly processor intensive stuff. My guess MSFT had to make a choice between 64 bit and additional features in the runtime, like them I would have put my money in the additional features.
The Gu gave an interview with Ars Technica where he talks about the plans that Microsoft has.