Got a good offer. And an immediate cancellation.
The T-Mobile MDA Vario IIIs impending release means I have a very sound alterior motive (other than my principles) to escape from my o2 contract. Phone companies are all crooks, it's well known and strangely well accepted so I feel no remorse over taking advantage of this opportunity.
I sent a cancellation email to o2 support (not the email listed in these comments) before calling them. The call resulted in my contract being reduced from £40 to £15 due to my relatively minimal usage... not bad at all. However, the email resulted in immediate calculation with no further questions asked. The latter is obviously preferable as I'll start my T-Mobile contract at the end of October and even £15 extra a month would make my other half frown.
It was clearly meant to happen, just a shame that T-Mobile in reality are just another crook suckering me in with promises of a shiny, shiny PDA phone.