The ATT plan requires a one year commitment unless you are a corporate user and they may have special agreements that allow you to turn it off sooner. I don't know if they will allow you to turn off the voice side.
Since you would not need a voice plan, the T-Mo plan for BB is $40 + $20 for international BB data and email, $60 in all. If you keep the $30 voice plan it's $70 in all, so you decide whether or not you want to add the voice plan or not for $10. You can turn on / off the international addon the day you leave and turn it off the day you get back by calling customer service. They will prorate the days you have it turned on/off. Without a voice plan the calls are charged at 20 cents per min + any additional roaming fees, you can call customer service via 611 when in the states. Another item to consider, if you have a voice plan with T-Mo and with an 8320, when you are near in wifi signal, you can call US numbers and only have the minutes deducted as those calls are UMA and will not be charged as international call back to the states. This only works on calls back to the US, calling China via wifi will have the international roaming fees.
I've used T-Mo without voice in China - wherever there is China Mobile, you will get your BB data. For voice I used the local sim card option with another phone and kept the BB strictly for email/data. Charges were as noted above the monthly BB data fee + the international package, that was it.