Short answer is no, Photo Stream is not a back up, it is a short term storage and sharing venue for a subset of your most recent photos. I use DropBox and a utility I found that pulls my photos and metadata out of the masters from iPhoto and drops them into the same folder structure in the DropBox photo gallery, works great. So after I import photos either from my camera or Photo Stream into iPhoto, I run this utility and push copies of the actual photo files into DropBox. This lets me have several things over just backing up the iPhoto library, it lets me share photos over the web like the old MobileMe gallery and gives me an actual back up of the actual picture file for using how ever I want in any application I want.