Are we there yet?
Looks like it. Initial tests are promising. Last night Xamarin released Mono3.1.1 which amongst other things included the PCL reference assemblies.
Creating a new Portable Library
Go ahead and create a new Portable Library. If you drop down the References folder you will see that System, System.Core and System.Xml are all in red. To fix this double click on the project and go to Build/General for the following dialog:
Make a selection there, click OK and now you should be able to compile :)
A barebones MvvmCross application with a Core PCL and Touch project compile and work with no hassles. Previously you had to jump through many hoops to get to this point.
I also added a Xamarin.Mac project just to test if I could reference my PCL Core and that also worked just fine.
Time to start using MvvmCross the way Stuart envisioned. I'll certainly be changing from a linked file solution to a PCL solution over the next few weeks. Hoorah!