One of the other nice things about Postgres is the pretty much annual release of a new version. 9.2 should apparently hit beta in the next week or so, which will bring some cool new features : http://archives.postgresql.org/pgsql-hackers/2012-05/msg0034... .
If you have a DBA that knows MySQL extremely well and doesn't need to change. If you have a existing system running well, and the future roadmap of your development doesn't make staying with MySQL difficult.
I am totally a Postgres fan, but there are many sensible reasons not to upset the apple cart.