Yes and No.
The sidekick II has 1800 only and not 900 bandwidth for overseas markets.
All of China uses 900 and Hong Kong uses both 1800 and 900.
Check out these links:
For a map of GSM coverage world wide - click on "roaming on the top tabs"
http://www.gsmworld.com/index.shtml
For info on the Sidekick II:
http://www.phonescoop.com/phones/phone.php?p=560
If you want to rent a phone to use in China rather than buyinhg a quadband phone like the RAZR or SLVR.
http://www.pandaphone.com/phoneshowroom.jsp
***JUST A SMALL EDIT***
SideKick bandwidth
GSM 900 / GSM 1800 / GSM 1900
(PV-100) / PV-108 model has GSM 850 instead of 900
As long as you DON'T have the PV100 or PV108 you're OK to use almost anyone in China. Sorry, forgot everyone in the world use the Yahoo Answers service and dont know where you are.