So if you have a business that is all Mac, say 100 machines, $29.99 U.S. covers the upgrade for all of them to Lion???
No. The App Store licensing terms are listed
here. Choose your country, follow the link to section C, then scroll down. In English versions, search for the heading "MAC APP STORE PRODUCT USAGE RULES".
In short, US$29.99 is the price for any of the following:
(a) Personal (non-commercial) use, on an unlimited number of computers owned or controlled by that person; or
(b) Commercial use, on an unlimited number of computers used by a single person; or
(c) Commercial use, on a single computer used by multiple people.
A typical business with multiple computers each used by one person will need to pay $29.99 per computer to upgrade to Lion via the App Store, and will need a separate Apple ID for each employee or computer.
There are
volume licensing solutions which will be more practical for all but the smallest businesses, but from the look of the Snow Leopard pricing (which are per seat), they are in some cases more expensive than buying the retail product. Volume licensing prices for Lion haven't been announced yet.