People overseas have asked if I can recommend a stored-value SIM package for them during their stay in Hong Kong, so instead of repeating my answer over an over I've decided to create a post. Before reading this post one needs to understand one thing about the mobile market in Hong Kong. It is very competitive and the rates and packages change frequently; around every six months. So aside from the following recommendations you should also verify my information when you arrive in Hong Kong.
Another thing to be aware of. Except for speaking to Smartone-Vodafone, don't use the phrase "Stored-Value SIM" to describe what you're looking for, as most of the other 6 carriers do not know what that means.
GSM Mobile without Data
If you're not using a GSM mobile phone with a micro-SIM card then you have many options. Any of the 7 carriers: PCCW Mobile, CSL's 1010, CSL's one2free, 3 HK, Smartone-Vodafone, China Mobile and China Unicom, in Hong Kong will have stored-value SIM cards for you.
GSM Mobile with Data
It appears that 3 HK and China Mobile are the only carriers in Hong Kong who are willing to serve overseas customers that want to use mobile data on their phone while in Hong Kong. With the former offering 3G Data connectivity.
GSM Mobile with 3G Data and micro-SIM
Even though Apple HK has three official carrier partners for the iPhone: CSL, 3 HK, Smartone-Vodafone, and a total of four carriers that sells iPhone 4 tariff plans, there is only one carrier (3 HK) that offers stored-valued SIM packages with micro-SIMs.
The type of SIM cards that 3 HK sells are those that can be either a regular mini-SIM or a micro-SIM (for the iPhone 4). Although, after you make the choice of turning it into a micro-SIM there's no way to go back.
Conveniently all carriers' store-value SIM packages are available at 7-11 so you can make your choice at the Hong Kong International Airport before you get onto the Airport Express for town.
Have a good stay!