Have you contacted AppleCare about this? If you purchased Lion less than 90 days ago, you get support. If you have any device with current AppleCare, you get support. Even if neither of these is true, if you are reporting a bug, I would think you get support. I know for sure you can walk in to an Apple Store (with an appointment) and get a half hour of "genius" time for this issue whether you have AppleCare or not.
I have never seen this issue. I do have one setting that may differ from yours. I set iPhoto to automatically create events for photostream so my photos never get deleted just because I didn't get around to dealing with them. I got events created for "november 2011 photostream", "december 2011 photostream" etc.
There are some more things I do differently that may help your situation. When I find the time, I open the auto-created events in iPhoto, export everything then re-import it. Once I'm sure I have them all re-imported, I delete the auto-created events which had original photos in them. This moves the files out to my NAS drive where I want the originals and gets them out of my iPhoto library so it doesn't get to be huge and possibly become unstable. I had LOTS of trouble with iPhoto back when I had a 67 Gig iPhoto library and I spent lots of time on the phone with Apple over the issues. I deleted that corrupted library and turned off "copy photos to iPhoto Library when they are imported". I now have an iPhoto library that tracks metadata for 200 gig of photos but the library itself is under 10 gig in size and gives me no trouble at all.
Apple might not admit this but iPhoto Library is not a proper place for originals of your photos!