I've seen it but I wouldn't recommend purchasing LR through the Mac App Store. At the moment, Apple doesn't offer upgrades on the Mac App Store so it means (unless Apple's Mac App Store's policy changes) that you'll have to pay full price when the next major release of Lightroom is released.
Tom Horgarty writes about that problem
here …however, there are a few important pieces of information you should be aware of:
• This is a single platform license for Mac only. (The boxed version and the ESD offering on Adobe.com can be installed on Mac or Windows)
• Updates will take place through the Mac App Store, not via Adobe.com for this version of Lightroom. When we update Lightroom for new camera support(about 4 times per year), the Mac App Store version may be released at a different time than the update on Adobe.com
• There is no upgrade pricing available on the Mac App Store for Lightroom customers who own Lightroom 1, 2 or 3.
• Because there is no upgrade pricing or upgrade validation currently available on the Mac App Store, there is no guarantee that upgrade pricing will be available to Mac App Store Lightroom 4 customers when Lightroom 5 and future versions of Lightroom are released.
• Lightroom 4 is available on the Mac App Store in North America only