I see the same thing as Boghetta. I use Aperture, not iPhoto. Once my photo stream hit 1000 and I started taking pictures with my iPhone, the very first pictures in the stream deleted, as expected.
BUT, I have 'Automatic Import' checked under the photostream settings in Aperture (same as what one can do in in iPhoto, I believe). This feature creates monthly projects called "Oct 2011 Photo Stream," "Nov 2011 Photo Stream," etc. and pics from the stream automatically end up here. BUT now that pics are deleting from my Photo Stream, their imported counterparts are ALSO deleting from within these projects. Even if I move early photos out of these projects and into my own named projects/folders, they disappear as new content is added to my stream.
This is a major problem for me. The only fix is to turn off Photo Stream and go back to manually importing my iPhone pictures :-/ And worry that all existing pictures that have auto-imported from my stream since Oct. 14 are going to vanish someday. Hence, I am hesitant to take pictures with my iPhone (with wifi turned on) until this behavior is resolved.