From TFA:
Unfortunately, some of these features are not what they first appear. The HTML control in Silverlight 4 is not a new embedded browser from Microsoft, but uses components from Internet Explorer on Windows, or Safari on the Mac, which means that the same content might render differently. The HTML control only works out-of-browser, and simply displays a blank space if browser-hosted.
The difference in rendering between IE on Windows and Safari on Macosx is a reality, whether silverlight is involved or not. The purpose of the HTML Control is to allow scenarios dependent on the HTML Bridge, the part of silverlight that blurs the lines and allows communication between the html dom + javascript and C# code, to run correctly when the app is hosted out of the browser. It's essentially a crutch to allow developers that want to use siverlight a way to leverage existing investments in web application development.
More seriously, COM automation is a Windows-only feature, introducing differentiation between the Mac and Windows implementations. Since cross-platform Mac and Windows is a key Silverlight feature, it is curious that Microsoft has now decided to make it platform-specific in such an important respect. Microsoft Office and parts of the Windows API have a COM interface, so access to COM makes Silverlight a much more capable client.
This is a fairly obscure feature, and I'm fairly surprised that it was included at all, but doubt it'll be of use to the vast majority of current and future silverlight developers out there. Like the html control, it's a crutch, to allow developers that want to use silverlight a way to leverage existing investments. The mantra I've heard out of the silverlight team is to focus on unblocking customer scenarios (scenarios they cannot unblock themselves) without compromising the overall feature goals (like keeping the runtime download small).
Nevertheless, Silverlight has crossed a threshold. It is now a runtime that has extended functionality only on Windows. That will not help Microsoft win developers from Adobe AIR, which has the same features on both Mac and Windows.
I don't think it'll matter. Any developer that is seriously considering using silverlight over Adobe AIR, but is then persuaded not to because Silverlight's Trusted Out-Of-Browser scenario has COM support on Windows and not on Mac is "Doing It Wrong". It's an edge case feature that doesn't affect Silverlight's over all "Cross-Platforminess".
Flame On.