i'm on 3.55 and i've been playing online since 3.56 was released.
you can either use fuckPSN which is a little complicated for the average user as it requires mounting your flash and replacing a certificate, or just use 3.60 spoofer to spoof your 3.55 to 3.60.
there are also methods to sign in on banned PS3s, though i still haven't gotten banned.