While I appreciate there needs to be something for managing Live@Edu/Exchange Online/etc remotely that dont work well with MMC, could you not just work with the relevant dev team to make MMC 3 work over HTTPS? I hate the way the "next best mangement thing" keeps coming along so often!
Also, what's wrong with AJAX anyway if you want cross platform? Remeber Linux has to wait for Moonlight to catchup and there are other platforms that have no Silverlight plugin.
Putting so much effort into a Silverlight interface, when there are plenty of more important issues with Exchange, seems like a waste of resource to me. How about improving IMAP performance, features and reliability, speeding up MMC 3, improving linux support for OWA, helping the Entourage team with their Outlook for Mac project to get feature parity this time around, implementing a standards based calendar server model to remove issues with sync from mutlitple clients, having true -seamless- cluster failover, making DPM 2007 reliable for exchange. These would all be higher on my list if you want to impress IT admins on the front lines and get people raving about Exchange and trying to convert the uninitiated.
Please consider working through the bug lists and architecture issues with extra resources before bolting on yet more new things.
PS PowerShell underlying at the core of every GUI feature (or a decent management API) is critical, people have to be able to automate when you have 20k users! Without this we wouldnt be using Exchange.