The Ms-PL is for all intents and purposes an open source license.
This is a diagram from the introduction to ASP.NET AJAX:
The left-hand side is what has been released under the Ms-PL. For the whole stack to work on Mono, we will have to do some work on the server-side assemblies that provide the support (the System.Web.Extensions assembly). It is likely a lot of work, but considerably less than the whole stack.
The AJAX Control Toolkit comes with 30 controls for common tasks, with a nice sampler of the controls. The Control Toolkit is actually being developed in the open, and Microsoft is integrating external contributions.
Nikhil has a retrospective on how the project came to be. A good chunk of information for those of us interested in the archaeology of software.
A big thanks to everyone at Microsoft involved in picking the Ms-PL license for these technologies.