Just an update with my recent HK experience.

The only no-contract micro-sim card with data for the iPhone 4 that I could find was with 3 <three.com.hk>. Card is rechargeable. Here are the details:

3G Rechargeable SIM Card
Cost - HK$198 for 3 months
Monthly charge - HK$18 (includes: unlimited wifi, unlimited in-network sms, and 50MB data)
SMS - HK$0.7 inter-network
SMS - HK$1.8 international
Data - HK$2/MB or max cap of HK$18/day or HK$168/month
Data Roaming - HK$0.11/KB or you can buy an unlimited data roaming daily pass (China or Macau HK$98; Asia or Europe HK$168)

My coverage was excellent in HK and Macau with the exception of some dead zones on the HK express to/from HKG and the ferry to Macau.

If you are a standard sim smartphone user, there is another choice, HSDPA Broadband Rechargeable SIM Card for HK$68, $98 or $298 face value. Data cap charge of HK$28/day or $298/month

The 3 staff at the airport shop in arrivals at the airport was friendly, helpful and knowledgeable.

Cheers