last year we tried the x10 cam.
It worked pretty good if the transimtter (TX) and reciever (RX) were in the same room. If we tried to drive out of a classroom and down the hallway we would loose signal rather quickly.
I would "assume" that in an open arena it would work ok.
We did notice a bit of EMI. (shaky video when driving certain motors). I'm sure relocating the camera would help.
One source that I have looked into for this year is
http://www.4atv.com/
as stated above the ATV band requires a licensed operator. But for $79 per RX or TX, it is an excellent price to get 2.4 or 1.2 GHz video over a greater distance than what the x10 cams can provide.
I'm just waiting to see if we have any "extra" money after all the ncessities are purchased (supplies, shirts, $$ for nats, etc :-)