I'm actually in a similar situation as you, but going the other way. I've just bought a new phone in the UK (it's on UK T-mobile, but is pay as you go rather than on contract), but I'm moving to the US in Jan 2008 for at least 4 years (PhD) and I've been trying to research the possibilities of using my UK phone abroad.
From what I've gathered (although I may be wrong), it is possible to buy international pre-paid SIM cards to use in your phone when abroad and you can then make local, national and international calls whilst in a different country for pretty good rates.
The process involves getting your current phone unlocked so it can be used abroad with the SIM card.
It may be possible to use your phone in the UK anyway without doing this (as long as it is triband), but the calls will be very expensive (to use my UK phone in the US it costs about $2 per minute to make a call and $1.50 per minute to receive a call!)
It might be easiest for you to terminate your contract due to moving out of the country and then sort something out in the UK when you get here. I don't know if you would be able to use your Sidekick 3 on a UK service, but there are some very good deals on contract phones here in the UK if you needed a new phone to use here.
It depends how much you are likely to spend each month, but T-Mobile UK as well as other providers (Orange, O2, 3, Vodafone, Virgin etc.) offer good deals where you pay a fixed amount per month, which gives you so many minutes and so many texts included and often you will also get the phone free (the cost of the handset is absorbed in the monthly payment).
For example, say you currently spend more than $50 per month, you could get a 12-month contract for maybe £25 per month (about $50), which would give you 100 minutes of calltime and 200 texts each month and you'd pay either nothing or a very small amount for the actual phone. Or if you went for a more expensive contract, you would get one of the top new phones free and also several hundred minutes and texts per month.