have you been looking at sharepoint (yeah, yeah, okay "moss")? wanted to add some ajaxified love to those new interfaces? or perhaps some of your own web parts.
take a look at some comments from mike ammerlaan on the topic. in a recent post, mike talks about integrating asp.net ajax with sharepoint (see he doesn't even use moss). there are some tips on extending the sharepoing web.config with the appropriate elements, yada yada.
with these two resources it should be enough to get you started -- or at least thinking.
Please enjoy some of these other recent posts...