The folks at Ajaxian just published the results of their second annual survey of Ajax developers.
A number of interesting tidbits – first, the sheer dominance of Prototype, Scriptaculous, Dojo, and DWR as frameworks (after that you start to move into the long tail):

From this, at least, it appears few folks are leveraging the commercial AJAX toolkits. (Of course, maybe those folks just don’t read Ajaxian?).
The platform usage graph is interesting too:

A full 50% listing PHP – but .NET does fairly well too. Atlas, the MS Framework for doing Ajax, only comes in at 4% of framework use, but .NET comes in at 16% of platform use.
They also note a full 25% of developers still say they are not using frameworks at all, and are working directly with the XMLHttpRequest object. While I can imagine a few edge scenarios where that makes sense, it seems absurdly high – are people really operating under the assumption they will individually create something better than these open source frameworks will?