Kurt Cagle has an interesting post on "Where is XML going".
One of his bold predictions is that SQL databases (by which I think he means relational DBs) are going to plateau and slowly be replaced by XML-based DBs. I find that a fascinating possibility. Certainly the emergence of XQuery finally provides a decent query language for XML (and it's interesting to note that it draws from or at least parallels many SQL concepts).
One question I have is whether the XML information model is in fact more expressive than the relational model. It certainly seems so, since you can easily represent a table in XML, but the converse is not so simple. XML contains implicit information about hierarchy, which must be supplied in a relational model. It seems like there should be some sort of information content metric which could be used to determine which model is more powerful. Although, perhaps the human factors are in the end more important.
Also, to me it feels like XML suffers from too much concern with syntax (whereas relational is a pretty pure data model). There's also the cruft of elements versus attributes, processing instructions, etc. All of these complicate the XML model without bring a lot more data expressiveness. Perhaps simple use patterns will emerge and we'll all just get used to ignoring this stuff (but it seems to me that's been said about SOAP as well, and look how successful that's been...)