im in the same boat as you actually.
Except i was looking for a bit more than GPRS, my phone supports EDGE and I found one cheap provider that allows for unlimited GPRS/EDGE on a pre-paid sim.
It's $138 HKD/mth for unlimited usage otherwise its about $0.04 HKD/KB.
That is by FAR the cheapest provider i've seen, no commitment or anything required, you just have to have a minimum balance of $150 HKD i believe in the prepaid account before you can sign up for that data package. It also states you ARE allowed to connect a notebook/pda/pocket PC to your cell phone to access the data network (which i have no idea about for all the other networks in hong kong)
They have a few different types of prepaid sim cards, all of them allow you to get that data package, then depending on what type of calls you make they range from less than 1 cent USD per minute locally in HK or 3 cents USD per minute back to Canada / United States.
Anyway, only downside i've read about them is they don't have the best network coverage, they do allow roaming so if you don't mind paying a little bit extra when you require it it will be fine.
Haven't found anything better so far...
http://www.peoples.com.hk/p_homepage_iso.jsp