This is what I've picked up several years ago and continue to use while traveling in H.K. and China - it's from China Mobile H.K. & retailed for $120 HKD or about $15.50 USD - with a starter stored value, can be easily refilled in HK or online here in the US (not all of the prepaid products can be done easily from abroad) - $50 HKD/less than $7 USD is good for another 180 days.

http://www.hk.chinamobile.com/p_prepaid_1c2n_en.jsp

There are obviously other products out there from variety of mobile carriers, and you can find some of them available for sale on eBay - contact/email the seller first & see if they can and/or willing to pre-activate the Sim card for you as part of the purchase, if they are based in HK or China region. You will then have a working p/p Sim card with an assigned number before you leave, and it will work in practically any unlocked quad-band GSM phone, available cheaply these days since many have migrated up to the iphone/smartphone lineups. This Sim card will work on the native AT&T and T-Mobile network in roaming mode - and it has optional mobile data access as add-ons for daily, weekly or monthly use (on 2G/3G). It wasn't designed for the iPhone/iPad or 4G network overseas but for basic browsing, email checking - this is fine for an inexpensivie quad-band GSM phone when one doesn't have access to WiFi or other data options.

I have not check out China Mobile's mainland website to see what's available as they should/do have prepaid options that extend to HK since many continue to use their devices while visiting/shopping & staying there.

These sim cards (maybe, borrow a friend/relative's unlocked quad-band phone if not buying one) are definitely far cheaper than paying AT&T or T-Mo for their sky-high rates to roam.

Also, see this thread for a related discussion & the Sticky for similar Q&A -
http://www.flyertalk.com/forum/trave...-question.html