AFIAA, there exists a port of Java for just about every OS that PostgreSQL supports, not that it should be the only reason for choosing it. Not that my vote counts, but I'd go for the java approach and be willing to code a lot on the interface, anyone else interested?
Anyone thought about wxPython? Much faster then java, can be distributed as a standalone executable on Windows. Supports Unix / Mac / Windows. Don't know if it supports more or less PG relevant platforms than Java. I have been thinking about working on this type of tool myself.