While Silverlight is certainly an exciting development, Microsoft has s serious credibility problem with cross-platform solutions. They killed IE for the Mac, Windows Media Player for the Mac, Microsoft Messenger for Mac is nowhere near feature parity with Windows Live Messenger, Remote Desktop Connection for Macs still PowerPC only and isn't on par with the version in Vista, Office for Intel Macs has lost VBA (and still hasn't shipped), Virtual PC for Mac is pushing up daisies...am I to seriously believe after this long trail of corpses that if Silverlight "wins" then Microsoft won't choose to kill it too for Mac?
Silverlight looks exciting as a .NET developer because I can transfer those skills to Rich Internet Applications (RIAs). But if I care about cross-platform at all, Silverlight looks high risk. Microsoft's track-record just isn't great. I would think they nearly need to publicly state that Silverlight will be developed cross-browser and cross-platform for, say, 10 years to ease fears that they aren't going to pull the rug out. Or instead of a pledge, port Visual Studio to OS X :-)