Old-client/new-server...issues?

I am working on my final 7.4 -> 9.0 upgrade. Data-type (auto-coalesce
and foreign-key), query and encoding issues are resolved. Dump/restore
is tested. But...

I have a few old client systems that are difficult/infeasible to update.
I know the recommended procedure is to update clients ahead of server
but what problems am I likely to encounter connecting to a 9.0 server
from a 7.4 client? The client software is primarily in C, Python, Perl,
Ruby and PHP with some Bash scripts using psql.

Note again - the database isn't using any data-types that didn't exist
in 7.4 and the queries themselves are tested OK with 9.0 so I'm more
interested in backend format and related issues.

Cheers,
Steve