A while back I wrote a post on an inventory of existing javascript autocomplete controls. I was writing an application that required one and found most of them to be unsuitable for what I was doing. I ended up using WICK (Web Input Completion Kit) an creating my own ASP.NET user control around it.
An article just came out in CodeProject, however that claims to do everything I was looking for. I haven’t had a chance to look at it yet, but when I do, I’ll let you know how it goes.
Here’s the article: http://www.codeproject.com/aspnet/AddingGoogleSuggest.asp