IIRC none have actually been sued, but one has been sent a letter saying they're violating a competitor's patent.
What made me start thinking about this idea was a combination of that letter, this story:
Here's a pledge, fuck software patents. I used blackboard in multiple schools and it was terrible, so screw anyone who tries to impede those who improve upon that system.
Given that PG cannot think of "anything that would have turned out for the worse without software patents", what-say-you we hold out for what is really needed, no software patents. That is, let's wait until this problem gets worse, get even more outraged, and effect our own change by getting businesses for whom we work to stop engaging in these vaine battles
http://k9ventures.com/blog/2011/04/27/modista/
and the fear that YC applicants working on education software have of the notoriously litigious Blackboard.