The below is all my opinion, I work for orange blah blah
To clear up some of the networky things in this thread that are a bit off (this is all I can remember, 90% sure it's right)
Three used to run their own HSDPA sites (pretty sure they are all upgraded to hsdpa now) and now have a radio network share with T-mobile which has seamless roam (eg you can't tell). This share is 2G & 3G. This was setup under the company MBNL (think this Mobile Broadband network limited or something equally crap).
Three used to 2G seamless roam with Orange until the agreement above - this recently expired which is why there were a lot of unhappy people unhappy with shock coverage change.
Orange and T-mobile obviously now have a joined 2G network (and soon 3G) but it's basic, there's no synchronous handover for example (can't move a call from one network to the other) Big improvements coming this quarter - if we arn't top of coverage I'll eat my hat. Orange joined MBNL BUT only in the sense that they are part of T-mobile's part of the share. It's still split 50:50 with three on one side, Orange & T-mob on the other. Once it's all fixed all three will "subscribe" to the use of MBNL's network equally. Ah, modern science. So wonderful.
So, we will end up with 3 effective networks in the UK by 2012: O2's, Vodas and MBNLs. 2G will probably be on it's last legs by 2015 assuming we all upgrade OK which will remove o2's/voda's lovely 900Mhz advantage. 3G all works on the same frequency for all networks so there's no advantage to any network in 3G coverage.