when I first used 3G on nokia e65, it shortened the battery life quite a bit with original firmware, so I forced phone to GSM only - this ability to choose network is disabled in Three-specific firmware, but my phone was unlocked with generic . I talked to someone in the business about this and was told that Three have ways to monitor whether someone appears locked to 2G and can, within terms and conditions, require that customer goes back to dual mode or 3G only.

if you're on 2G, there are things which won't work - they have to pay Orange quite a bit for roaming, so I'm told, especially for data, which explains the above.