Moneysupermarket.com have published a helpful table (scroll to the end of the webpage) comparing UK iPhone 4 deals, in order of total cost over the contract period.
Tesco seem the cheapest.
Not that I'm necessarily rushing to get one myself.
I'm not normally an Apple person. Their admittedly very clever marketing and ingenious brand imaging haven't succeeded yet in winning me over as I'm don't care that much about fashion and I don't like Apple's over-controlling and locked down approach, which takes away users' own power and control - from the way iTunes was originally DRM'd to the hilt (despite the availability of some iTunes files now in non-DRM form), to Apple's insistence on vetting all third party developer iPhone and iPad apps to their own arbitrary moralistic and possibly monopolistic criteria (as witness what happened initially with Simon Maddox's excellent and consumer-championing 0870 app) and taking an unreasonably long time to do it in too, to their refusal to support Flash (e.g. for viewing YouTube videos), and effectively trying to stop cross-platform mobile development. I think the US were right to start anti-trust probes into Apple.
But, having tried a friend's (older model) iPhone, whose soft onscreen keyboard is much more accurate and faster to use than the Android G1's soft keyboard, and having seen the huge range of iPhone apps available for free or at very reasonable prices, I have to say I'm just a little bit tempted…