Then you would need a professional digital asset management tool, not a consumer freebie photo album.
"My Volkswagen Rabbit speedometer only goes up to 140 MPH. What happens if I go faster than that?"
"Then you have much more pressing problems to attend to than the speedometer, friend"
if you have 250K photos, there is no way in hell you should be using iPhoto, regardless if it can handle it or not. i got rid of iphoto for iView media pro after i got about 1000 photos.
1) Use multiple iPhoto libraries. The limit is per library but the number of libraries is not limited. But you can't search across libraries
2) Use Aperture.
#2 is a much more reasonable solution. I was at a three hour presentation a couple days ago were Apple gave a great Aperture demo. They sad there are some Aperture users who have over a million images in there Aperture library. If you have this many images you likely also need Aperture's much better catalogging abity and it's ability to handle off-line images and do backups. $299 is not a bad price if you have over 250K image to store