I would use mysql, was just curious as it seems vB is planning multiple db support for the future, and given this project is being written from the ground up it could be on the plans to already support it.
well it is always good to have plans to support additional databases in the end. especially now that oracle owns sun microsystems which owns mysql and oracle is messing up a lot of things that were good at sun. and mysql could join that list. from what i have heard postgres is a good alternative though i have never used any other database system other than mysql.