In reply to EwanR:
I live in the French Alps and am entering several races this year. However in previous years I've just got my UK GP (we had a GP service at the office) to do it. They basically check you're not going to die and sign it.
However, every time I hand in the medical form at the event (Etape, ultras etc) they hardly even glance at it, so I would say you could easily fake it. Its not like they call up the Dr to check*
There is often a pro-forma one you can download and print out on the event website (which is it, out of interest).
Anyway, this year I'm going to go to the doctor as I need to register with a French GP now that I live here. I've heard that its the French Drs who take it too seriously (they're a nation of hypochonrdiacs and people like to get their money's worth) but I'll let you know in a week or so. Anyway, I'm as fit as I've ever been so I guess if there is anything that comes up, I should take it seriously.
*Disclaimer: I'm not advocating faking it, just saying that it hardly seems to be worth the paper its written on as evidence of being fit to compete.