Is it possible to build a Portable Class Library with Monodevelop?
Update: this accepted answer is now very out of date.
The current situation is that you can do quite a lot with just a few small hacks - see http://slodge.blogspot.co.uk/2012/12/cross-platform-winrt-monodroid.html
The near future situation is that fairly extensive full support is in active development - http://slodge.blogspot.co.uk/2013/02/the-future-is-portable.html
I don't believe it is - I think the current PCL implementation from Microsoft is quite "closed" in the way it's implemented.
Also, be aware that if you are targeting MonoDroid and MonoTouch type projects then these are not officially supported and they don't fit into the exact defined portable library profiles - e.g if you try to include System.Net, then you'll have problems working with MonoDroid.
That said... using some small "hacks" like http://jpobst.blogspot.co.uk/2012/04/mono-for-android-portable-libraries-in.html it is possible to get PCLs working across non-Microsoft projects.