What's the catch? (Mobile provider Q)?

(Btw, I'm not very clued up on the mobile provider business, so you'll have to bear with me if I say something stupid, or this entire question is stupid.)

So I'm looking for what is called a SIM only deal, which I hope to get if I buy a new iphone 4s. So I've come across a deal with Virgin, which gives unlimited minutes, texts and data for just £15 a month. I'm also with Virgin Broadband and TV, so apparently I'm applicable for some kind of discount as well? Not sure how that works either, if someone could explain. So this all sounds very nice, but surely there's some catch? Is there something I'm missing? This entire question probably sounds very stupid to those of you who know a thing or two about phones, but I really know nothing… So please help? What am I missing here? And can anyone explain Virgin's discount policy?

Virgin's discount policy is very simple… You have home Internet or whatever with them, you get a discount. With regard to the actual contract, there's a catch. Virgin media don't support iPhones. Especially ones with nano sims.