Not a deal for a lot of people, but if you are on the 50% friends and family discount on T-mobile the iPhone 4 is now available on it, so no need to lose your discount. As they made you do prior to this going live.
The 3GS and the 16gig iPhone 4 is free, the 32gig iPhone 4 is £49.99
The catch is.. its only on the £60 plus tarriff (Bascilly everything is free) so that means you pay £30 a month. (unlimited text, calls and data (with 1gig fair use policy)
This may well go cold, but for people on this discount its a good if you want the iPhone 4 compared to any other network or offer I can find. So no need to leave Tmobile for a better deal