[URL=http://arxiv.org/abs/astro-ph/9811018]According to this guy, it is.[/URL]

He's going to have a TV show on Science HD, God help us all.

I'm about half way through his paper but it seems like the 4 sided cube guy. The speed of light is variable, he says, but we cannot measure its variances through experimentation because time cannot be measured without a variable as well (even atomic clocks suffer from this). I wonder how the XKCD guy feels about him, I personally think it's the kind of pseudoscience along the lines of String theory that makes for good TV but doesn't actually [I]do[/I] anything, unlike cosmologists like Hawking who help refine our ideas about black holes and bring us closer to a unified field theory.

Tl;DR Joao Magueijo says energy does not equal mass times the speed of light squared.