I've not been one to worry too much about whether a tune was Bluegrass or not. A quick, off the top of my head answer is anything played and/or sung in the style of Bill Monroe and his Bluegrass Boys and Bill's contemporaries such as Flatt and Scruggs, Stanleys, etc. This definition leaves a lot of room to maneuver. Oh yes, count me as a Bluegrassholelic too!

RMan