By making the following changes iPhoto will delete the images imported *after* asking if you are sure you wish to delete the files. As always, make a backup prior to any operation that could concievably result in lost data.
defaults write com.apple.iPhoto.plist DeleteFilesAfterImport\? YES
defaults write com.apple.iPhoto.plist DeletePhotosAfterImport YES
To restore your settings:
defaults write com.apple.iPhoto.plist DeleteFilesAfterImport\? NO
defaults write com.apple.iPhoto.plist DeletePhotosAfterImport NO
[robg adds: As noted, be very careful if you decide to change these settings -- once gone, you'll have great difficulty retrieving the files!]

