The point made about EPO is pertinent. There are those who will always push at the boundaries of the rules (written or unwritten). In a way it's how things move forward - the classic example from athletics is the Fosbury Flop in high jumping, it wasn't banned as no-one had ever considered it until Dick Fosbury worked out that his CofG actually passed under the bar so he didn't need to jump so high.

I've not used GPS devices in races but then I've not actually raced for a few years and back then they weren't exactly svelte or easy to use. I was never going to trouble the sharp end of any race anyway so wouldn't have gained much if anything if I had used one. The middle third of most races are a procession, you aren't going to get lost.

If you are at the sharp end of a race then you aren't going to be wanting to look at a watch every minute or so to check if there's a shortcut coming up and you'll be breathing so hard you won't hear a beep unless it's the volume of the Land's End fog horn!

Potentially it's more use to the tail end charlies who might not know the route and are out of sight of the group in front when they make a crucial turn, I'd argue for the safety case in this instance and wouldn't be bothered whether such use was declared or not, I'd rather they were out and enjoying themselves.