Dave Winer points to a blog posting by Erik Speckman calling Google Desktop Search a disappointment. One complaint that Dave shares is that Google search only works with certain file formats. He wants to be able to add plug-ins for file formats it doesn't already understand.
Well first of all, this is a BETA technology. I think it's a tad bit premature to call it a disappointment just yet. It's already far better than any other free desktop search utility out there.
Secondly, it may well be as extensible as Dave and Erik would like, but as a BETA product, we can't be sure yet because such interfaces are not published. Scott Hanselman started digging into it a bit and noticed various dlls used to implement searching across IE and Outlook. That suggests that there might be a plug-in architecture they will expose later (perhaps after getting feedback during the BETA trial). In any case, Scott's peek inside is a worthwhile read.
UPDATE: Erik notes in reply on his blog that I have a valid point regarding Google Desktop's BETA status, but that the point of a BETA is to gather criticism. I agree wholeheartedly and I think he gives some solid suggestions to the Google team. The title of his reply is "Google Desktop Search BETA is a disappointment" (emphasis mine). That clears it up for me. :)
If you're a Windows, Office, and Outlook user like me, I think you'll find the BETA worthwhile. If not, Google has some work to do to make sure the final product isn't a disappointment for users like Erik and Dave. Hopefully we'll see an extensibility framework come out for it.