That's an interesting question, moonjen. A great many tunes have become bluegrass standards that didn't start out as either bluegrass, old-timey or country tunes. some that come to mind are:

Fox On The Run --- Manfred Mann.
The Bells of St. Mary's --- movie theme.
Limehouse Blues --- old jazz standard.

McGrath, you posted before I finished or I'd have added a reference to that album. My copy still has the $0.97 price tag on it from Rose Discount Records on Chicago's Wabash Ave.

A tune often crosses into the bluegrass genre when a banjo player or mandolin player gets inspired and decides to work it up. Once it's been "ported" the other band members can build on it pretty easily.

I'll be interested to see how many other people come up with.

      - Mark