Since T-Mobile and Orange merged to become Everything Everywhere, there have been plans to allow users of either network to roam on the other for free. This has been in place for 2G voice and SMS services since October last year, but now sharing of 3G signals has started.
I received a text message earlier from T-Mobile to let me know about this starting. All that was required was to reboot my phone.
As indicated in my text message, further information about this is available from the T-Mobile website.