I got a question at a recent MSDN Roadshow regarding using the Atlas AutoCompleteExtender with a list that already exists on the client (ie no call to the server). That’s not the scenario the control was designed for so the simple answer is no – that control requires a web service to function. Of course if you already have the list on the client then it should be possible to tackle this with some relatively straightforward client-side javascript and potentially wrap this up as an Atlas behaviour to re-use across multiple TextBox controls.