While looking for info on the Champlain Valley Festival in Vermont, their site linked to another neat site being compiled about music festival of all sorts of different genres:

http://www.festivalfinder.com/

It doesn't have a lot of folk festivals yet, but seems to be growing.

I work on crew for several festivals co-sponsored by the National Council for the Traditional Arts; two of their biggies are:
The Washington Irish Festival at Wolf Trap, Virginia, on the Sunday before Memorial Day. This is a great Irish festival featuring some of the best traditional musicians and dancers from the Irish-American communities around the US, Ireland and related musics from Cape Breton, Quebec and more.
The National Folk Festival - this one moves to different spots in the US every three years, and features a mix of nationally known acts and performers from the local area. There's always a mix of a lot of different knds of music; this year in Dayton, Ohio, we had Don Walser and his Pure Texas Band (country/honky-tonk), Bruce Daigrepont (Cajun), Samba Ngo (African), Sabor Latino (salsa/merengue), Valahia (Roumanian dance band) and lots more - you get the picture. Next year's will be in Dayton again, then on to a new city.
The NCTA sponsored festivals and tours tend to concentrate on "Roots" music from particular ethnic/regional communities, both acoustic and electric, so you get to hear a different mix from the performers that do the "Folk circuit" of Old Songs, Mystic, Eisteddfodd, etc. Not saying one is better than the other; I like being able to hear both!