Re: Do we have to return them?
You are not currently required to return it. It might be a good idea, however right now they're merely providing incentives to do so - offering refunds, waiving fees, restoring upgrades, etc. It is possible they will destroy the functionality of the devices that aren't returned, but no one has officially said that they will or even hinted that they might. If you do decide to keep it, please don't sleep with it next to your face and I hope yours isn't one of the ones that we read about.
I agree with the first three points of dsignori's post and the fifth is obvious, four I'm not as sure about but it wouldn't surprise me at all.