Yes CAPPA is legit, it's also a certifying organization for doulas, childbirth educators, etc.
Currently there are three organizations that I know of that certify birth and postpartum doulas - DONA, CAPPA and ALACE. CAPPA and ALACE also certify childbirth educators.
They are just different schools of thought for the most part. ALACE doulas do things DONA forbids, whereas CAPPA is more like DONA. Here are the links to their websites:
DONA - www.dona.org
CAPPA - www.childbirthprofessional.com
ALACE - www.alace.org
Lamaze also certifies educators - www.lamaze.org
I know this is more info than you wanted but I figure if nurses work in OB, and have to work with patients that hire doulas - it wouldn't hurt if nurses knew what the doulas were all about, and the different organizations that certify them.. plus some of the organizations are always adding new certifying programs.. so another one might come out with another lactation education program.
Anyways, hope that helps!!