Have open-source contributors ever been sued for patent infringement in their contributions? What was the outcome?
I do not know of any direct case where a contributor was sued for its contributions for patent infringement, but there are likely quite a few cases where users of supposedly infringing FOSS code were sued. The best example that comes to mind would be Microsoft vs.TomTom about the Linux FAT support.
There is also a case where RedHat (and may be Novell/Suse) were sued of a workspace switching feature, likely related to either Gnome and/or KDE that both Suse and RedHat have heavily contributed to.
Also, some of the patent wars that involved Android adopters vs other mobile device manufacturers where at least in part related to FLOSS.
The thing is that patent situations are often settled... It is quite possible that MPEG-LA is demanding licensing fees from users of FFmpeg for instance, and you would never hear about this.
Now, my understanding is that patents are about ideas and their usage: I would tend to consider (but I am likely a misguided non-lawyer) that the mere fact to release FOSS software that may implement a patent may not be infringing. Instead, if I were to start using this FLOSS code, then I might be infringing.