For the same price as my current plan for my iPhone, which currently includes unlimited data (in reality, it's "fair use", but I've apparently never been unfair), O2 will in the near future give me a whopping 500Mb per month, and charge £10 per 1Gb over the limit. I reckon that's almost good enough reason in itself to not upgrade to an iPhone 4. Doing so would cost me at least another £20 a month in data charges alone (plus the cost of the handset, plus £300 to buy myself out of my contract, plus the deeds to my house etc). Of course, I'm sure they'll whack the prices up for existing contracts soon enough, too.
That shiny new version of Skype with 3G calling no longer looks quite so attractive, does it?