Some of these mobile phone cashback deals have been the subject of unprecedented levels of complaints to the telecom watchdog. One of the problems is that to get them, you take out two contracts - one with the phone supplier with whom the cashback deal exists, and the other with the airtime provider, who often have NO liability over the cashback of the phone itself.
So .... if the phone supplier goes down, you won't get the cashback, but you're still stuck with what may well be an expensive and unattractive airtime contract once the cashback element has been taken out, with no way to get out of the airtime contract.
But med2003 is right too - Canon are currently running a rebate program on a number of cameras, lenses and accessories, and have done so for several years running. There have been some complaints over this sort of scheme being slow to cough up, but I know some number of people that have availed themselves of that Canon deal quite successfully ..... and I'm about to.
To come back to freddie's question, as far as I'm aware there's no legal problems with these in the UK, but I'm certainly not aware of it being all that common. I think it's not not something that's done very often. Maybe it's been tried and not proven popular, or maybe it';s just a marketing technique that hasn't been extensively tried here.