I'm looking into building a large-scale J2EE application that really doesn't have much more functionality than storing/retrieving data and then formatting it either to the user interface, a predefined "form", or to XML based webservices. There might evolve into a need for relatively complex relationships in the future though.
So my question is, bearing in mind that my biggest concerns right now are low-cost (its a startup), scalability (its a "cloud" app), integrity of data (data corruptions would be very very bad, moreso than usual in this case), and security (information needs to be kept confidential), what are some good starting databases to use?
I've looked into PostgreSQL and it seems to have come a long way but I'm being tempted away by DB2 Express-C and a few other free commercial versions. I'd prefer to stay away from Microsoft SQL Server because of its lack of multiplatform support (albeit I will admit its a pretty good database).
Does anyone have any experience along these lines?