This new Apple support document reveals that, starting with iWork ‘09, the physical retail boxed version no longer requires a serial number. iWork is now the same as iLife which has never required a registration code.
HOWEVER, if you download the trial version and then decide to register it you WILL be sent a serial number that must then be used to enable full use.
So for one less serial number to track, grab the boxed retail version.
… And Stay Away from Pirated Copies!
Some of the copies of iWork ‘09 floating around the file sharing networks also contain an added bonus of an evil nasty trojan horse application! So keep it legal folks.