Me too, Aperture did not erase the SD card in the card reader, neither when I imported all images nor when I imported selected images:
AP 3.2.2, MacOS 10.7.2, MBP 13"
I'm not too concerned about it - usually the camera knows best how to erase the card safely.
I don't have an answer.
I do have a suggestion . Standard "Best Practice" is to erase images from cards using only the machine which recorded the files in the first place (in this case, your camera). Furthermore, it is generally advised to reformat cards, rather than just erase pictures. This, too, should be done by the camera. (If you share cards between cameras, you should reformat them whenever moving one from one camera to another.)
I keep enough cards to cover at least two days work. After each shoot I import into Aperture and keep the files on the card(s). As cards fill, I replace them with my "least recently used card". When I run low on a card, I replace it. Every time I replace one, I reformat the card I just replaced it with.
This way, in addition to whatever file backup routine I have on my computer, I also retain the originals on the cards for at least a day and two nights.