I AM.........................COMPLETELY............... BLOWN AWAY BY THIS!!!!! WOWWWWWW.... the #1 reason I stopped investigating Xamarin was that it requires you to build 3 different UIs for your solutions. No longer with Xamarin.Forms. I am totally getting rocked with this new feature, as it is precisely what I envisioned last year that needed to be created in order for Xamarin to be feasible. And Xamarin has done EXACTLY THAT. VERY IMPRESSIVE!!! I am even more impressed that Xamarin has actually made a MORE FUNCTIONAL XAML PARSER THAN WinRT!!! That is, in one release, there is support for the IMarkupExtension interface, something that has been lacking from WinRT from its initial release.
I am speechless. Well, not entirely, because I do have a few questions. 
(I do apologize for my excitedness, as it has been a long painful road since WinRT has been released, and the lack of true Xaml -- with true markup extensions)
1) Tooling. As mentioned in another thread, there appears to be limited support for tooling in Xamarin.Forms solutions. To be more precise, if I open the HelloXamarinFormsWorldXaml solution, and go to the StackLayoutExample1.xaml.cs file, InitializeComponent does not resolve as a symbol. I use ReSharper, so I am wondering if this has something to do with this, and/or if there needs to be a fix from ReSharper (I will be posting in their forums as well).
2) When the heck did Xamarin.Forms release?!
3) How does Xamarin.Forms integrate with Universal Apps? That is, it appears I can only make a Windows Phone 8 app currently with Xamarin.Forms. I am curious about the support of this and if there are plans to enable support for Universal Apps.
I cannot stress enough how awesome this is. The lack of IMarkupExtension support in Windows RT was not only a headscratcher, but a heart and deal breaker. It is one of the most powerful features of Xaml, and has been PAINFULLY absent from the latest technology stack. The X in Xaml is for EXTENSIBLE, after all.
Kudos to the Xamarin Team. Wow... this is very impressive. I have been spending the past year and a half in HTML5 hell. It looks like I have finally been saved.