If you are an Orange or T-Mobile customer (Contract or Pay Monthly) you can use the other networks 2G masts from 5 October 2010.
You need to register you mobile number at:-
Orange - http://www.orange.co.uk/share
T-Mobile - http://www.t-mobile.co.uk/share
The agreement is part of their new parent Everything Everywhere. Initially the sharing will be via opt in at the above addresses - in 2011 it will be automatic and include 3g (ideal then for Android and other inferior Smart Phone users).
Need to register, then I believe it will be a case of Manually selecting the other network - ideal for T-Mobile customers who have poor coverage but use their phones in areas of good Orange coverage and vice versa.
Does not start til 5 October 2010 so please do not comment that it doesn't work - wait til at least 8am although suspect goes live shortly after midnight - no doubt some Orange or T-Mobile staff can confirm.