I've noticed a few projects attempting to tackle the issue of Bluetooth connectivity from common code in Xamarin.Forms.
At Evolve 2014 we released a beta version of the Monkey.Robotics component which contains a cross-platform Bluetooth LE abstraction that can be accessed from Xamarin.Forms apps.
I also have two Xamarin.Forms samples that use Bluetooth LE:
Because the component doesn't know about PCL projects, you just need to remember to reference the core robotics library yourself from the PCL. See the apps above for an example.
As mentioned above, the Bluetooth LE stack is open source and currently still under development. Consider it a beta that is still undergoing testing. We are also still working on the Windows Phone implementation of the API.