This is interesting ... and worrying for iOS developers using in-app purchase in their apps.

Two iSO developers - James Thomson and Patrick McCarron have reported receiving threatening legal letters from an as yet unnamed third party.

Thomson used in-app purchasing in his PCalc Lite app (PCalc is one of those apps that I use all the time)while McCarron used it in a game called Shanghai Mahjong.

Neither are willing to discuss the matter much as they want comment from Apple's legal folks, but to me this smells like a patent troll.

Other iOS developers using in-app purchasing are understandably worried, such as Gedeon Maheux:

Seems there's a patent troll about who doesn't have the courage, confidence (or money perhaps) to go after Apple so is instead going after indie developers.

Apple needs to do the right thing here and indemnify developers why use the iOS SDK (Software Development Kit) to create apps from patent issues relating to the SDK.

[UPDATE: It is now being suggested that Lodsys LLC is the company behing the legal threats.] [UPDATE 2: Dev Matt Braun who makes the MASH iOS game for kids has also been hit with legal threats.] [UPDATE 3: Here's what intellectual property activist Florian Mueller had to say in an email: "While the patent system works quite well for large players with deep pockets, little 'indie' app developers don't have the resources to defend themselves against such threats. In most cases they don't even operate under the legal umbrella of a limited liability company, so in a worst-case scenario they could be financially ruined for life if they lose a patent infringement lawsuit." Serious stuff!] [UPDATE 4: OK, some more information. I've receiveed information that suggests that it is indeed the Lodsys that is behind these legal threats to developers. The patent in question here is 7,222,078 (which is listed on Lodsys) 'Methods and systems for gathering information from units of a commodity across a network.' And yes, it is as vague as the title makes it sound.]

Any readers who happen to be devs having a bad Friday 13th thanks to being hit by legal threats? Let me know either in the comments of via the contact form here on ZDNet.