Working with v8.3.4 DB using v9.0.1 software

Hi:

I'm working with 2 different PG installs on 2 different linux servers, one running v9.0.1 and the other v8.3.4. The older install is "frozen" for the project (which is nearing completion) for environment stability reasons. This is out of my control. I cannot upgrade v8.3.4 on that server. But I would like to be able to use psql v9.0.1 on the older DB. It seems to work, with a warning that some features may not be available or work right.

My question has to do with backward compatibility, v9.0.1 and v8.3.4. Can I use v9.0.1 software (psql, pg_ctl, etc...) on the v8.3.4 DB with confidence?

Thanks in Advance !