In Leopard Mail & a bunch of the iLife apps, you can open up a Photo Browser that lists Apple apps & the photos they contain. I know that Aperture, iPhoto, and Photo Booth show up there, and I'm not aware of any other potential sources.
Is there a documented or known-but-undocumented API for becoming a source in that photo browser? I know that any app can open a photo browser. I want to know if my Lightroom photos will ever, ever appear there. An extremely preliminary googling turns up nothing, so I assume the answer is no.