This question came up at the Public Sector Developer Conference in Anchorage, AK.  I'm trying to answer all customer questions like this via my blog so everyone has the benefit of the answer.  There are a number of ways to do it, but my favorite (because it so darn easy) is to use an ASP.NET AJAX control extender Dan Wahlin built.  I love reusable controls that make common tasks easier:).  The details and download for the extender is here.