It appears that both the AT&T Samsung Focus S and Flash will be capable of Internet Sharing. Or at least that's what AT&T's Support Websites are indicating.

Support sites for both the Focus S and Flash contain instructions on how to turn on your personal hotspot with these new Windows Phones. Both instructions do note that there is a prerequisite to have a compatible rate plan. We're not sure if that means you have to belly up for a DataConnect plan or if the DataPro 4GB plan will do the trick.  Personally, I'm crossing my fingers that the old unlimited plans will work.

You can find the tethering information for the Focus S here and for the Flash here at AT&T Support.

Thanks goes out to Ryan for the tip!