I'm not typically a math-oriented person, but I decided I ought to do some looking into the possibility of a varying speed of light. I found an interesting article which unfortunately is not going to be available on the web to everyone. Ellis, George; Uzan, Jean-Philippe. "c is the speed of light, isn't it?" Am. J. Phys. 73 (3), March 2005, 240-247. I'm sure that's not the proper reference format for physics, but it has all of the necessary information. Basically a varying value of c is a popular idea among secular physicists as well as creationist scientists. This article examines the significance of varying c, starting with the question of "Which c is varying?" They describe four different types of c's: cEM, the electromagnetism constant (speed of light in a vacuum) cST, the spacetime constant (the actual c in E = mc^2) cGW, the speed of gravitational waves in a vacuum--whoa! I didn't know there was an official consensus on this, but these authors say that typically cGW = cST according to general relativity, and if we change the value of c this relationship may change. cE, the Einstein space-matter constant, which is equal to cST by general relativity, but will not be if c is changed. This is a list of very stringent criteria! No one has yet formulated a theory of a varying speed of light that is internally consistent in these ways. This is something that will need to be done if any of the c-decay based creationist models are to be taken seriously.