Ya know, Einstein himself wasn't sure about his E=MC² formula.
In th ebeginning he but something else in the back. He added the greek letter lambda. He called it the "universal constant". He called it his biggest mistake. But, towards the end of his life, he started thinking about it again, and decided that it wasn't so foolish after all. He just never got round to publishing again. But quite recently (I saw this all in a documentary some while back), some professor started thinking about it too. And he's starting to reach the conclusion that the lambda was indeed correct. Now, what is really important is that nobody is sure whether it's a constant-constant like Avogadro's number (6,0221367 * 10²³) or a constant dependent on certain criteria (like gravitation). That's what I meant by light speed not being a constant.