My understanding is that CSL is the only company that will give you HSDPA access for the iPhone 3G on a prepaid card (at HK$50 for 1 day, HK$178 for 5 and a higher fee for a month).

People (China Mobile) only has GPRS, while PCCW doesn't list any data services (other than pay-as-you-go Wi-Fi) for prepaid.

Thanks for the pointer on the 7-Eleven in the arrivals hall!