AJAX in ASP .Net - Missing Sys.WebForms.*

So - don't ask me why, but I've been dabbling with AJAX & .Net 2.0. Oh, the shame of it. I just spent two days trying to figure out why my UpdatePanel controls were doing a full refresh of the page instead of only updating the area they encapsulate. It turns out that the required script libraries (.axd files) were not all being rendered into the page. In particular, I was missing Sys.WebForms.PageRequestManager. It took a further two hours to find the following (seemingly irrelevant) blog post:

http://blogs.technet.com/stefan_gossner/archive/2006/01/24/418005.aspx

In a fit of desperation, I tried his suggestion for fixing a completely unrelated problem, and lo, it worketh. Just comment the following out of your web.config:

<xhtmlConformance mode="Legacy" />

Enjoy.