Don't use iPhoto for long term storage of photos.
I don't use iPhoto (or Picasa etc) for long term storage of photos. I store all my photos in its native form, like Word Doc files, on external HDs and DVDs. I import the photos into iPhoto only when I want to create a slideshow, organize before emailing them out etc.
my workflow with photos
-Use free Mac utility app called Image Capture to download photos from camera onto Mac HD. Do not use iPhoto to download photos.
-Use ExifRenamer to rename photos to date/time it was taken. Seems not necessary but crucial for headache free management of photos. Each photo will have unique file name AND you can tell where/where it was taken just by looking at the file name. No need to check thumbnail images.
-Copy the photos to External HD (or burn DVD or both) for backup. Backing up with Time Machine is NOT enough as older backups are erased eventually. You could use same HD for both Time Machine and storing photos long term. You should plan on swapping out this HD every 2 or 3 years. Put it on your planner, Mobileme iCAL, etc.
-Import photos into iPhoto if you want to browse through them or create book or create slideshows to share.
-Lastly, get 'iPhoto Buddy' so you can use multiple iPhoto Libraries (on internal AND external HDs). Putting ALL your photos into 1 huge iPhoto Library ON your internal HD is not smart imo. Maybe create a dedicated iPhoto library for each year. Keep it on an external HD.
-After making sure older photos are on external HD/DVD, delete them from your HD. I do. My Internal HD is just not big enough for all my photos.
Now you might say that's no unApple with so many seemingly manual steps. The fact is Apple can't do anything about users filling up HDs with thousands and thousands of 5MB jpg files or video files. You as a user have to determine what to keep where. No amount of technical solution will fix that, unless of course you have unlimited internal HD storage, which no one does.