Thomas Borchert wrote:
Unless you don't believe that the speed of light is a constant in vacuo.

What's there not to believe? Anyone using GPS cannot deny Einstein - it
wouldn't work without relativity.

Relativity has certainly been well-tested and (as you say) widely used.
However, there is serious discussion taking place concerning the
possibility of a variable speed of light as an alternative to inflation
during the early phases of the big bang.
See
http://www.iop.org/EJ/abstract/1475-7516/2003/07/004 and
http://www.arxiv.org/abs/gr-qc/0209014