SUN Solaris is as far as I know an OS, the question is, what database does it run?
From the documentation:
Which databases do we support?
Our platform is fully tested on SQL Server 2005, Oracle, PostgreSQL, Informix and HSQLDB.
As long as it offers the standard API interfaces that the database you want to use offers, it shouldn't be a problem.
Everything is fine on any database in Solaris, except loading a model in mendix platform at system start up. It can takes a lot of time, due to the shortage of SQLite interpretation in Solaris. So Mendix will use his own Java function. But the performance will be ok after it is up. Not sure how Mendix solve this in 2.5. I will try it next week ;)