updated 01:00 pm EST, Tue December 20, 2011
Mainly hurts users of first-, second-gen hardware
People still using versions of iOS 3.1.x on an iPhone or iPod touch have begun encountering problems with the App Store, according to threads (1, 2) on Apple's support forums. On a basic level, pages on the store may contain broken layouts and missing content. More serious are problems with actually downloading apps, such as having to use the Update All command, instead of having the flexibility of updating apps individually.
Should that fail, people at least have the option of updating apps through iTunes on a computer. Apple has been made aware of the issue, but doesn't appear to be offering any solutions so far. Since the glitches are explicitly related to the App Store, they may imply that Apple has made some server-side changes that broke compatibility with older releases of iOS.
The impact should be limited to owners of first- and second-generation iPhones and iPod touches. Anything released afterward either has newer firmware pre-installed or is fully compatible with iOS 4 or 5.