I've been puzzled by this issue for a long time...
My brother used to ask me why there were no people in my images, and I told him that posting pictures of identifiable persons could mean trouble if I didn't have permission or a signature.
Only the last couple of months I posted a few images with identifiable persons... but I'm still not sure if that's okay, legally speaking...

From what I've heard, any identifiable person that could be seen as the main subject, or as one of the main subjects of an image, can legally demand that the image should not be published or used in any situation or circumstance, for whatever reason that person may have. If the image is already posted or published, that person can legally demand or impose the withdrawal of that image in any possible way or published form...
But I know there have been some issues (at least one of them on the Flickr site) where such an image was used for commercial reasons without permission of the depicted person and/or the photographer, due to some dubious usage policies on certain photo sites...

EDIT: the term 'any identifiable person' is not totally correct, because I think people 'in the public domain' (celebrities, politicians in function, etc) are in a different category... ;o)

We do not see things as they are. ǝɹɐ ǝʍ sɐ sƃuıɥʇ ǝǝs ǝʍ