Sorry, Belfast, Ry Cooder would have been a dreadful mistake, much as I like him. They've picked a few totally appropriate musicians like Riley Baugus (awesome Round Peak clawhammer banjo player and fine fiddler) Tim O'Brien and Dirk Powell (superb Old Time musicians as well as great in other genres - bluegrass and Cajun respectively. They featured in the original excellent Cold Mountain CD from a few years ago), as well as some fairly inapproprate ones like Stuart Duncan - a superb bluegrass fiddler who'd be incapable of sounding like an old time fiddler if you ran over his hands with a road roller, and Alison Krauss - well I'll wait and see before condemning her outright, she is pretty able.
The success of Oh Brother was partly due to the fact that relatively unknown extremely able musicians were used - they should have done it again with a new set, of whom there are stacks. They only had to spend a week at Clifftop and the job of finding them would have been done.
Cheers
Ray