The Modis Team has released Sedna 3.2, an open source native XML database for Windows and Linux written in C++ and Scheme and published under the Apache License 2.0. Sedna supports XQuery and its own declarative update language. Chnages in this rleease include:
I haven't had the opportunity to use Sedna myself, but I've heard good things about it from some folks who have. Likely worth a look.