CentOS: PHP 5.3 (version 6), PHP 5.1 (version 5)
Debian: PHP 5.3 (stable/squeeze), PHP 5.4 (testing/wheezy)
So yeah, ATM 5.3 on most servers (well, if you're using precompiled binaries from the default repos that is) and thus at home for development until 5.4 is adopted in those two major...