I think you misunderstood Kirk; you can crop without performing a save as. This crop, however, is not applied to the image data (i.e., no information is lost and you can always revert back to the original). If you want to really apply the crop and remove information, you will have to perform a save-as.
I for one like this behaviour as PM for me is not an image editing tool. At the most I would use the crop functionality to give me an indication of the result if I would crop it. The real crop I would perform in my image editing software (e.g., Capture NX, Photoshop, etc.).
Hope this clarifies things a bit,
