Good topic Mike!
I think that most people who don't like Bluegrass, either simply don't understand it and haven't been interested enough to give it a chance, or they think of the older Bluegrass as banjo and mandolin heavy (especially banjo) and somewhat clanky sounding. I don't mean this in a bad way and don't want to offend anyone but I believe that's how people perceive it. That style and our predecessors are why we are here today doing what we're doing. I also think it's important to go back to our roots and learn as much as we can about the early bluegrass.
Today's bluegrass to me is more evenly spread out between all the instruments, especially allowing more guitar breaks. This makes me happy, not sure about everyone else.
So to answer your question, I would have them listen to anything modern but still sticks to the roots of Bluegrass. AKUS would be a perfect album to start with. I asked my wife your question (she's not the biggest Bluegrass fan but does like some of the later groups) and she immediately answered "Dailey and Vincent"! Brilliant I thought. I also think the Alan Jackson Bluegrass CD would be a good one along with Della Mae if she wants to hear an all female group.
Ken's right. Seeing a live show can give someone the bug. This happened to my Mom. She hated Bluegrass my whole life, until she went to a live show and saw Joe Mullins and Radio Ramblers (there's a good one that sticks to the roots) and now she's a dedicated follower.
Once they get the bug, they realize the real talent in Bluegrass music and develop an appreciation.