Is cross-platform typically not an issue for your projects, or do you just target the lowest common denominator supported by the DBMS types you're targeting?

Cross-platform? Sure! Cross-DB? Not usually. Take, for example, Krang. It's proving to be very portable, due partly to the fact that MySQL is very portable. Installing MySQL is so easy that it's hardly a barrier to adoption.

Supporting multiple databases in an application is very rarely worth the trouble, in my opinion. I say choose a good free database with wide portability and damn the rest.

-sam