I live in China, and it is very easy to purchase a prepaid sim, and top it up, here. They cost 50 - 100 RMB (8-16 AUD).
To use blackberry though, you'll need a blackberry service, which obviously you'd need to subscribe to, so it isn't really feasable for a short trip.
Another option is to sign up to a VOIP service such as PENNYTEL, which will give you an Aussie local number, which you can then forward to the chinese prepaid number for a flat rate of 8c per call. You could then forward your aussie mobile to that number, and only be paying domestic call forwarding rates +8c.
pennytel also have a bb app which means you can call any australian landline for 16c flat rate per call from a chinese mobile.