The reason I don't have a nectar card
At least I'd have to get one for them to do this, so it's better than phorm.
Shoppers will have internet adverts displayed to them based on their offline shopping habits in a new scheme being developed by internet publisher Yahoo! and customer loyalty scheme Nectar. The two companies will link their databases in a bid to better target consumers with relevant adverts and to improve the tracking of ads' …
Article says quite clearly: it is opt-in. Also every store card you can imagine does the same. That's the reason you get those funny little discount vouchers printed with your receipt. And somehow they always have something you bough recently (or been buying regularly) and you probably would like it.
If you really want to be anonymous, use cash only and wear oversized dark sunglasses, as maybe they can find something on store cameras about you. It is just simply impossible not to leave trace anymore.
Making this opt-in only is a great way to allay everyone's fears, but this is just the thin end of the wedge. The scary part is how innocent the whole thing must seem if you don't think about it too deeply.
What possible harm could there be in being presented with adverts which are more relevant to your interests? Surely it might even make the adverts less annoying? It's only when you think through where this will inevitably lead in five or ten years time that you see the sinister side.