Phil,
My initial concerns would be:

1) You go to a show where there are 500 PF users carrying these devices. What happens? Does it just give you a big list of who is there? I can image it being next to useless unless it can pinpoint the position of each user it recognises and tell you the direction they are in?

2) Is it removeable from the car to carry around at a show? As you are rarely in the car..

3) Security? Someone gets a hold of a device with the sole aim of driving down streets and findind garage containing cars from people in specific owners clubs? it could be used to find someone then read up on them on the net and find a lot of personal details etc. Also imagine if a sexual assault occured if a female member was tracked using this device.

Initially it sounds like a great idea, but possibly a bit of overuse of technology to find a solution where a name sticker would suffice in many situations. At £90 I think it is too expensive for what it does. Don't get me wrong it would not be a cheap system to develop and operate, but it doesn't produce £90 worth of gains for me from what I can see. It is hard work getting members to pay £20 towards the running of the site, so £90 for a gadget to help identify them when not on the net seems a little over-prices IMHO.

Hope that helps a bit.