Photons must have mass because light can be asorbed or bent around the gravitational field of a black hole...   i remember reading somewhere that astronomers had observer a binary star much further away than they should have been possibly able to. and this had been due to a blackhole deep in space acting as a giant lense - bending the light from a much more distant object and refocusing it.

and another reason - a mirror can be pushed by a beam of light hitting it - this was demonstrated by a fan inside of a vacuum sealed glass tube - when a light beam was directed against the blades it will start to turn.  forget the name of this instrument.

Interesting stuff though.

Clint