Recently, I tried to export 800 images (<1G) from iPhoto 4.0.1 and got the message that I had insufficient disk space to complete the operation despite having 20G free on my HD.
I checked the internet to look for others with this problem. There were several comments about iPhoto remembering the size of the original HD on which it was installed and several workaround solutions but no solution till now...
Turns out the solution is easier than the workarounds (e.g export to alternative HD etc). All you have to do is to erase the iPhoto preference file ~/Library -> Preferences -> com.apple.iPhoto.plist and all is good again
One of many fixes for what appears to be several problems. See thread for other fixes, or see my postings. As of May 04 I used Panther Cache Cleaner's most extensive cache cleaning function to resolve a recurrence of this problem.