It seems KnowRoaming has now launched. Card is US$30 (and US$5 shipping to HK) and upon activation you get US$15 credit. Rates are US$8 per day for data roaming (it seems this is a discounted rate with US$12 being their regular rate). 55 countries for data roaming.

This is clearly much more expensive than getting a local sim card, but I guess there is the convenience factor, especially if you are going somewhere for a weekend. I know in the US you have the likes of TMO with some very competitive roaming rates, but in HK our carriers are charging anywhere between US$12 to 24 a day for daily data roaming (and this is based on HK time, not time of your destination).

http://www.knowroaming.com/

Am tempted to get it to try out. I also do note that credit is valid for 15 months.