Basically, early this month, Iranian forces began to shell Kurdish targets within the border of Iraq. The central Iraqi government has made some noise about the attacks, but the government is weak enough, the PKK dubious enough, and Iran otherwise diplomatically occupied that the story hasn't really gone anywhere.
Also contributing to ignorance about the story: that area of Kurdish Iraq and Kurdish Iran does not seem to be particularly inviting to independant reporters. All versions of this story I've read rely on duelling third or fourth-hand accounts. The Iraqi regional minister declares ... The Iranian spokesman denies ... and we readers are left to worry.
And now Turkey enters the picture, massing significant numbers of troops along the border (but some might just slip over, here and there) to combat the PKK.
This very decent DKos diary provides the maps: Kurdish ethnic nationalism is no respecter of borders. We're going to hear more stories about the northern Iraq frontier regions, and I don't think they're going to be happy ones.
Update: And here's a worrying analysis from Damascus-based Sami Moubayed: it sounds like Tehran and Ankara might be talking about their problem in common... [via nadezhda.]