I put an example online that will show the current beta version of Ajax.NET Professional. Try the page http://munich.schwarz-interactive.de/default.aspx [1] with your Windows Mobile 5 device. You will see two alerts, one showing the current time of the web server, the second one will return two properties of an PersonX object in a List<PersonX>. Would be nice to see if we get it running on older Pocket PCs, too.
So, the is no difference made for Windows Mobile 5, everything is standard Ajax.NET Professional, simply add your AjaxMethod to the method you want to call and invoke it from the client-side JavaScript.
I hope to release this version on Sunday!
Note: only the default.aspx is currently using the new library.