I just spent a while confused because I was getting no results from SPSiteDataQuery and it turned out that for the query itself I had accidentally used an "And" element with only one "Eq" sub element which is, of course, invalid. Instead of throwing an exception it just returned no results so I was a little thrown for a while. I tried a completely invalid element name too and that didn't fail either so I recommend being very careful when developing your queries.
georgebonney