Now that the iPad WiFi+3G version is available and reviews of the device starting to fill the Internet. It appears that the iPad is not locked to the AT&T network.
The information about how to activate the AT&T 3G tariff plan on the iPad is a bit unclear. Screen shots of the Cellular Data Account screen and the process after the required information is entered are shown by iLounge, but how these information are validated and how some people are able to activate the iPad on T-Mobile data network is unknown.
Presumably activating the iPad on any cellular networks other than AT&T will not allow the self activation of tariff plans from the iPad, similar to the lack of Visual Voice Mail for iPhones on non-Apple partner carriers.
So in theory, to use the iPad WiFi+3G in Hong Kong all one has to do is to get the iPad from US, sign up for a cellular data only plan from one of the carriers, and then follow one of the many instructions online to resize the carrier SIM to the Micro SIM specifications. Good Luck! If you planning to do just that please leave your experience in the forms of comments below.