So far none of the solutions provided address the real problem, our iPhoto libraries are growing bigger than our hard drives.
If you properly format and migrate your library to an external disk you can keep buying bigger disks, which is great for a desktop but ***** for a laptop.
If you properly format and migrate your library to a disk attached via the local network you will only have access to the library when you are on that network.
Sure, you can view items you intentionally keep in Photo stream, but you can't keep everything in there if you're running out of local disk.
The backup offerings out there like crashplan and backblaze only back stuff up, you would have to restore it and re-import it into iPhoto to view it, losing any tags, faces, places or other metadata.
The only real solution is "something" that virtualizes your iPhoto library across your local disk, a backup device in your network, and an archive location in the cloud. IPhoto wouldn't be aware of all of this, only the "something". The local disk would hold the iPhoto library metadata database and new and recently accessed files, the backup device would hold everything else. Files would be migrated from local disk and backup disk based on a narrow set of rules you set, including % or Gb of space to use and last accessed file age. The cloud destination would be something you pay a regular fee for to hold a copy of everything in case of disaster.
If, while at home, you try to access an older file that has been migrated to secondary disk, it would pull it from your backup device on your network.
If, while away from home, you try to access an older file that has been migrated to secondary disk, and you're connected to the internet, it will pull it temporarily from the cloud service.
Things like this exist but only for large enterprise systems.