o2 has begun to upgrade there services. There is nothing noticeable as of yet, but in the next few months there will be a slight increase in speeds for EDGE and 3G.
for those who have the 3GS they will obviously have an increased connection due to the phone itself. The sharing of masts has been going on for ages, you bounce of other cellular carriers mast all the time. It's not roaming but it's just what happens. Sharing is probably one of the best ways to increase reception in certain areas. The big carriers i.e o2 voda t-mobile and orange if you like them- need to upgrade there towers/services to be one step ahead when better smartphones come out in the future.