As far as 64-bit browsing is concerned Silverlight 5 will have a 64-bit plugin.

The major issues I have with Silverlight at the moment is in the mobile arena. Windows Phone 7 is built on Silverlight so there's no issue there, but iOS and Android are left out in the dark at the moment. There's rumors of official plugins for both of those OSes and apparently Mono is working on an implementation as well.

This is a huge shame as I really like how easy it is to separate concerns with Silverlight using the MVVM pattern.

If you're seriously looking at Silverlight I would highly recommend you check out the Prism library from patterns & practices. The features you get from this library for building loosely coupled composite applications is amazing.

With the lack of mobile support we've currently suspended production of Enterprise apps with Silverlight and are moving towards Html 5 and knockoutjs to handle the MVVM implementation.