I guess the best way to mark something as yours is to individualise it. It makes it special, personal and more, far less likely to be stolen. Plenty already apply this principle to luggage and vehicles. It helps if the customizations are fashionable too.
Take a look at this: CrystalIcing - isnt that brilliant? Its a service (or DIY kit if you prefer) to have your mobile or i-pod done up in swarovski crystals, to your own design. Just choose your colours!
Makes me want a mobile phone just so I can bling it up!
On the up side, I guess, even if I left it on a bus, no-one is going to buy a hookey mobile with someone else's design on it, nor one thats been scratched to death to try and get the crystals off. What do I mean, up side, is there any other?