Umm, have you tested this? Deleting an image from the Photo Library (as opposed to an album) does indeed remove it from the hard disk, at least on my Mac. Try the following:
* Duplicate an image.
* Figure out the duplicate's filename (actually a number. Double-clicking to open it in another application for editing, as set in the Preferences window, is an easy way to do this if you don't have the Show in Finder script).
* Do a Sherlock search for the filename. You should find a couple of items in the iPhoto hierarchy starting with that number (a data file, a thumbnail, and the original, at least).
* Switch back to iPhoto and delete the image from within the Photo Library.
* Switch back to Sherlock and perform the same search. Every time I repeat these steps, all the files found in the previous search are gone.