Bracelet a good idea, mobile is not!
Replacing cash and wallet by a chip wrist band is a good idea (at least until you get mugged for it, but you could get mugged for your wallet etc anyway); and as it stays around your wrist the risk of losing it, having it falling from your pocket etc is reduced.... so that's a good idea.
Integrating it into mobile phone might have some advantage in every day life BUT festivals are clearly not the best place to implement it.
Being a regular festival goer/volunteer, the number of mobiles lost, stolen etc is quite impressive (the #2 item just after wallets). Any sensible festival goer will try to avoid having his mobile with him 24h/day (leave it in your car or in lockers -if provided as at Glade Festival-), and having to take it out every time you want to move, eat, drink, pay, etc... is increasing the risk...
Wrist bands are the way to go!