If you run into this problem, like I just did when updating from the RC to RTM, here is what you need to know. You'll need to set the new "EnablePageMethods" attribute on the ScriptManager element to true. Pretty simple, but I don't think this was made obvious in the migration guide.

It is in the documentation which you can find here:

http://ajax.asp.net/docs/tutorials/ExposingWebServicesToAJAXTutorial.aspx