Actually.... it isn't, and never was.
I read in the New Scientist that even though the speed of light is represented scientifically as the letter "C" - it was proven some time ago that the speed of light is altered depending on what it's traveling through - which is obvious when you take the time to think about it, because even photons have mass.
There was an article quite some years ago whereby scientists managed to reduce the speed of light down to a mere 38 mph iirc, a fact that's always stuck with me.
google fu shows this link has the gist of it:
abcnews.go.com
amazing what you can learn while waiting at the doctors!