This trick works with the last version of iPhoto ('09), but could work with previous versions as well. When you remove photos from an album, they remain in your photo library. Here's a way to see which photos don't reside in any album, and to then place (some or all of) them in the trash. Here is how to proceed.

First, create a new Smart Album named (say) Photos which are in no album with the condition Album -- is not -- Any. Next, select the newly-created Smart Album, and select the photos you wish to delete from your library. (You could select them all with Command-A, if you wish.) After selecting the photos to delete, click on the Photos entry in the Library section of the sidebar. Note that the photos you have chosen remain selected.

Now press the Tab key, which will move the focus from the left panel to the main panel without disturbing your selection. Finally, choose Photos » Move to Trash (or press Command-Delete) to delete the photos.

Naturally, this procedure also works for trashing photos which are in any given album.