While the latest RC of the the MVC Framework does include Microsoft's Ajax script libraries and JQuery 1.2.6, they're not accessible without manually inserting the script includes yourself.  In order to get up and running with the scripts, you'll need to include the following script references in your frontend code:

<script src="<%= Url.Content("~/Scripts/MicrosoftAjax.debug.js") %>" type="text/javascript"></script>

<script src="<%= Url.Content("~/Scripts/MicrosoftMvcAjax.debug.js") %>" type="text/javascript"></script>

<script src="<%= Url.Content("~/Scripts/jquery-1.2.6.js") %>" type="text/javascript"></script>

I like to include these scripts in the header tag of my master page(s), that way I can set them once and then forget about them.  If the footprint from the size of the scripts is an issue, you can always just manually include the scripts only on the page(s) that need jquery and/or ajax functionality.

 - Colin