Surfing the blog world with Feedster, and have come across a few more XML database links:
Berkely XML DB by Sleepycat sounds interesting. Has no mention of XQuery, though, which is a big drawback for our needs... (here's a development weblog for their xml database)
NeoCore XMS sounds great, but rather pricey (advanced XQuery, integrated XSLT, etc...)
Here's a thread on Slashdot about Open Source XML Databases... The thread includes this link to a weblog about xml databases, but it seems rather dated (last post was Aug 2002).