But for a photon, doesn't the spin vector always point in the same direction as the momentum vector - and therefore, shouldn't the magnitude of a photon's helicity equal it's spin magnitude, i.e. [tex]\sqrt{2}[/tex] [tex]\hbar[/tex]?

The spin vector is always at an angle to the propagation vector, such that its component in the direction of propagation is [itex]\pm \hbar[/itex] and its magnitude is [itex]\sqrt{s(s+1)}\hbar = \sqrt{2}\hbar[/itex].

In theory, one might expect that the photon could also have a spin projection of zero. However, apparently this would require that the photon have non-zero rest mass (which it doesn't), so a zero helicity state is not observed.

If somebody can explain why a zero spin projection is ruled out by relativity in more detail, I would be grateful.