Can't find any information on iPhoto's plugin development for iPhoto 11. Did Apple abandoned iPhoto SDK effort completely?
I can't answer that officially, but it sure seems like Apple abandoned it. The SDK/sample code used to be readily available on the developer site, but it's now pretty difficult to track down. The bright side is it can still be done with the old sample code they used to provide assuming you can find it. The down side is that when it's hard to find like that, it's likely Apple won't be supporting it any more. Of course, maybe they're going to adopt a new plugin architecture, but I won't be holding my breath.
I found this link to an iPhoto Exporter plugin code example: http://code.google.com/p/slideshowexporter/ I think whoever posted this one to google code modified it to meet their needs, but it looks like some of the original code is still in tact and might be a good starting place for you.
I can confirm that the Apple example code works with iPhoto 11. I just don't have an official link.
Good luck.