Did Planck think that light was a particle or a wave?
AFAIK Planck, like all his contemporaries, believed light was an electromagnetic wave (which it really is, the way it's seen now is that it's just that electromagnetic waves in general don't behave quite like how we thought).
Now, the thing about Planck's radiation law is that it implies light was emitted in discrete quanta of energy (c.f. the [tex]h\nu[/tex] term in Planck's law with the [tex]\Delta E[/tex] in the Boltzmann distribution).
But (as opposed to how it often gets described) Planck did not actually put any deeper import on that fact. He didn't really assume there were photons, or that energy/light was quantized (rather that, the 'oscillators' as he called atoms/molecules, emitted light in quanta).
There's an http://physicsworld.com/cws/article/print/373" [Broken] here that attempts to set the record straight on Planck's contributions.
I think it was Einstein, who realized that light was made out of particles.
From the http://en.wikipedia.org/wiki/Planck%27s_law: [Broken]
Separate names with a comma.