We actually support anything with a vendor-supported Java Runtime Environment and frankly, recommend against windows .... all you need is a minimalist Linux install with a JVM. We provide an application server, although you can use your own ...
We run in, but do not support, OpenJDK. We sell enterprise software, if we have a P1 and we nail it down to the JVM, there is not much the customer can do with OpenJDK, is the stance .... Imagine some 20 000 headcount corp has a P1 with a bug in Oracle or IBM JVM, I am sure Oracle/IBM will not patch it over a fortnight ... whereas in OpenJDK, you might be able to contact one of the maintainers to fix it ... handing over a Kraft envelope in gratitude ... ;-)
You could maybe get a support contract from Redhat for OpenJDK in which case we will support you, not sure how that goes, though.
We support all main databases (DB2, MaxDB, MS SQL Server, Oracle, Postgres, Sybase), and as said above, any platform/OS combination that has a supported 1.6+ JVM. OpenIndiana is my OS of choice.
We recommend Postgres for small to medium installs, Oracle RAC for really big ones ... recommend against SQL Server and Windows for obvious reasons.