vigaga
I don't think they "forgot" this feature (and I don't think it's "absolutely urgent" myself, but there you go...) You have no hope getting it included in the next release unless you tell the developers that you want it: iPhoto Menu -> Provide iPhoto Feedback
the way around by dragging the picture to the desktop, using a tool like graphic converter, reimporting it, deleting the unflipped photo can't be the way we will have to live with for all future....
You can set Graphic Converter (or any image editor) as an external editor in iPhoto. (Preferences -> General -> Edit Photo: Choose from the Drop Down Menu.) This way, when you double click a pic to edit in iPhoto it will open automatically in GC, and when you save it it's sent back to iPhoto automatically.
Rather less complicated than the export / flip / import option.
Regards
TD