The best world roaming SIM I have found (and have) is GeoSIM
(<--- click the link) . It is the best, IMO, because it is the only one that offers 30 second/6second billing (of the Isle of Man numbers. You can also get Estonia, Iceland,and Lichtenstein world roaming SIMs, but, they cost FAR more to call from the US than UK numbers)..
It is a normal UK cell number, and roams for free in 50 countries. Unfortunately, HK (40c a minute incoming) and China (65c a minute incoming) aren't 2 of them. But, a lot cheaper than T-Mo. You order online, and top up online. Outgoing calls from those 2 countries are maybe on average 60-90c a minute. But, the initial 30 second billing makes it nice to call someone to tell them to call you back, so it only costs half a minute.
to find out more of these things.
That all being said, local prepaid SIMs are REALLY easy to get in HK, and I think also China (I use my GeoSIM or HK chip there anyway, as I only spend a day or 2 in China at a time). If you subscribe to a Skype In number, and do some creative forwarding, it is, in the end, only 2c a minute to forward your T-Mobile phone to your local HK or China SIM !!!! (meaning, you forward your T-Mo to the US Skype number, and then forward your Skype to your Asia mobile number.) So, someone would call your US T-Mo number, and you only pay 2c a minute to receive the call in China on your cell !!!
That cheap eatins' !!!!
Of course, that ALL being said, unless you get an additional cell while you're there for the local SIM, you lose your Blackberry abilities with the Pearl and local SIMs.