I had an phpBB 3.0.0 board with GZip compession enabled and it was serving GZipped pages. But after upgrading to phpBB 3.0.1 the pages are no longer compressed even though GZip compression is enabled in the Server settings.

URL : http://animorphsfanforum.com/

Afterwards, I performed two fresh installations of phpBB3 :
Unmodded version of phpBB 3.0.1 : http://animorphsfanforum.com/test/
Unmodded version of phpBB 3.0.0 : http://animorphsfanforum.com/phpbb/

Both are are GZip enabled, but GZip compression is happening only in the phpBB 3.0.0 installation.

I'm ready to provide admin login details in case you need it.

PS : Even the phpBB community board is not serving GZipped pages even though it says GZip compression is enabled in the debug output.