There has been a patent troll going around contacting iOS developers who are offering in-app purchases, informing them of patent infringement. FOSSpatents has put up an article detailing what iOS developers should know about it. It also mentions (note that this are educated guesses, not facts) about how Apple and other companies including Google who also run similar app store might react. Do yourself a favor and read it even if you are not utilitzing in-app purchase today.
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