To enabled Ajax on your SharePoint site, you have to modify the web.config file as instructed here http://asp.net/AJAX/Documentation/Live/ConfiguringASPNETAJAX.aspx.
The last 2 parts can be skipped if that doesn’t apply to your case.
The good news is that this web.config file can be safely copied over in deployment. Actually it can be used in other sites as a starting point as well. So far the only thing we modified our web.config is to add some SafeControls. So the web.config files aren’t much different from each other anyway. (Update on 12/18: This is not always true. Make sure you don’t have extra entries in the SafeControl list that you haven’t installed on the target server. Otherwise you will see error when you add new webpart)