Unfortunately, if you do a fresh install on a Mac, then try the sequence "File->New->Portable Class Library" then that library will not build. The build gives lots of errors about "predefined type XXX is not defined or imported"
I think this will probably be easy to fix - it will probably be down to the PCL reference assemblies - but until some brave leading-edge adventurers have worked this out, then it might be wise to "wait" and not to install the 4.8.0 version
Hopefully soon this will get resolved - then we can async/await :) :) :)
Please RT if you think @XamarinHq should test File New PCL Project builds before they release any version as stable— MvvmCross (@MvvmCross) July 25, 2013