Actually, that's incorrect; Sun is a backer, but hardly a major one.
A quick glance at Ohloh's contribution metrics for PostgreSQL (http://www.ohloh.net/p/postgres/contributors/) shows 17 committers with over 100 commits. If you start matching nicks up to the PostgreSQL contributor profiles (http://www.postgresql.org/community/contributors/), you'll quickly see that only one, Peter Eisentraut, works for Sun.
Actually, it's interesting to note that of those 17 top contributors not a single company employs more that one top contributor.
If anything this is good news for PostgreSQL, then: Oracle can easily acquire and steer the course of MySQL development, but shifting PostgreSQL priorities by financial means would take the acquisition of Sun, IBM, UC Berkeley, and over a dozen more companies.