C# lets you make very powerful UIs with very little work (thanks to WPF) which most importantly feel quite native. WPF is also the only widespread framework that I know of that lets you have pretty much any kind of control inside another control (well, other than HTML that is -- the problem with HTML is that you lose the native feeling).

That being said Mono supports System.Windows.Forms (the poor predater to WPF) so there's always that. Or they can code against the (now dying?) Silverlight which actually works on Linux ala Moonlight.

But really, the best user experience is the one that feels the most native. So on Mac that would mean building a Cocoa client, on Windows WPF, on Linux Gtk/Qt (but then again who actually pays for Linux apps?)

Going with a one tech that supports it all just has a higher probability of having something that supports everything poorly, instead of something that supports one thing very well.

He revealed during the talk (after a very small amount of prodding) that it was an Outlook extension (since people on Windows are willing to pay for software... out of company budgets).

The brains are in the cloud though, apparently.

Not really, as long as they don't use Windows API directly and rely on .NET Framework they should be able to run it on MONO.

See Mono.