if you are looking to build office applications in Silverlight such as an mail client, word processing, et al check out http://divelements.co.uk/silverlight/.  Yes the following screen shot is actually running in a browser using Silverlight.  Very nice.  Add Silverlight 3 to mix and you can run both in browser and OOB in offline scenarios and go so dynamically given the new network awareness APIs in SL3.

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