[bcbrown]

I'm looking at Griffiths right now, and 9.3 Electromagnetic Waves in Matter states (emphasis in the original): "Since [the dielectric constant] is almost always greater than 1, light travels more slowly through matter - a fact that is well known from optics."

I'm still confused. How is it an inaccurate simplification to state that the speed of light in glass is lower than in a vacuum?

[eternauta3k]

I wouldn't call it inaccurate at all. I think it's a perfectly good model. You can explain the phenomena otherwise using a more detailed model, but you don't need to.