Northwest is definitely doing it on CRJ's, I've seen it a couple of times. It's one bag into the cabin, everything counts. This doesn't include rollaboards that have to be gate checked anyway, this is actually into the cabin. You should have seen the horrified older women in the gate area when the gate agent tells them they have to gate check either their purse or their small tote bag (both of which would fit under the seat or the overhead), they did not have a rollaboard. I knew about it because of threads on the NW board, so I was prepared.

http://www.flyertalk.com/forum/showthread.php?t=740402