I just did this over the weekend. AS PDX-LAX then AA LAX-IAD. I checked in with AS in PDX and had to pay the $20 checked bag fee. They transferred the bag from AS to AA at LAX without a problem. When you check in, make sure you tell them the continuing flight and the reservation code. Your bag will be checked through with no problems, but YMMV. Good luck.

Of course this is dependent on there being a DL-AA interline agreement, which I don't know for certain.