The Mainsoft developers have been busy implementing the core foundation for ASP.NET AJAX. They usually do all their testing on Grasshopper.
Recently System.Web.Extensions got added to the default build in Mono (I do not think this will make it into Mono 1.2.5, you will need to use the code from SVN for that) but we had not really tested it directly with Mono.
Onur Gumus posted a How To: ASP.NET Ajax with Mono document today.
For doing native development in Unix this is a bit cumbersome (due to all the new stuff that needs to be added to the Web.config file; Should we put most of this in the standard machine.config?) but if you are moving an application, the Web.config should have been produced by Visual Studio already.