There was recently a folk festival near here that was sponsored by Front Porch Music. While I didn't get the opportunity to attend the fest, I was reading in their newsletter about who played.

One of the bands was Weavermania, and in the "bio" of the band members, I came across this quote...

'Mark Dvorak takes Peter Seeger's role. An award-winning recording artist and performer, Mark has been called "a folk singer's folk singer" who follows unerringly in the footsteps of Woody, Lead Belly, Big Bill, and Art Thieme.'

I don't know that this says that Art is "famous," but it definitely seems to say that he is well known enough and good enough that any semi-serious folk fan should recognise the name.