You can still do this on US, DL, CO and AA. I've flown US, DL and CO since 9/11, and seen people doing it on my plane. A friend flies AA every month with her 1 year old, and has continued to do it, no problem.
My guess is that UA also allows it, and I would think so would NW and SW. Do'nt know about the others. Best bet is to call your carrier.
Anne