Implementing AJAX in Sharepoint
2 answers
It's possible to build SharePoint to work with MS Ajax controls - I've done this before, but it requires various changes to the web config and some hacks in the parent web part code - something that you are unlikely to be given access to a production system if you working with a SharePoint designer.
JQuery is probably the best option - you can put the script in a document library and it will make javascript easier to work with.
Calling standard sharepoint web services from javascript works pretty well - I recently posted some code examples on my blog that might help you.
http://tqcblog.com/2009/05/04/sharepoint-discussion-with-jquery
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