MySQL would be good solution, if it came with windows
No really.. Two major problems with mysql are:
1. It's quite heavy database
2. Most people can't install it
It would be possible to add 'optional' support for it, but it's awful lot of work + minor change in database would obsolete existing db...
I actually started thinking of making seperate daemon (which runs in same process with io) to handle userfiles.... ofcourse it isn't as efficient as reading userfiles directly from disk, but it would be more extendable