FOSS Patents has put together an excellent write up on what iPhone, iPod touch, and iPad developers need to know about patent litigation threats, like those Lodsys began making last week against apps like PCalc and expanded upon today, sending letters to Icon Factory among others.
Lodsys has explained their side of the argument, and according to The Guardian, Apple's legal department is actively investigating the situation. But to be clear Lodsys is just an example -- there are many, many patents floating out there and many companies assembling portfolios with perhaps the exact same intentions.
While users shouldn't be worried about patent companies suing anyone for using apps, users should be worried that it could become prohibitively expensive to produce the apps users if every holding company under the sun suddenly wants a cut of every app in the App Store. (Insert your own "first they came for PCalc's upgrade button, then they came for..." cautionary cliché here.)
Read the entire breakdown via the link below, it's a good one.