David Bowie is sidelined by a bum ticker and Britney had the doors to her Hotel Onyx closed by a bum knee, but the worse thing that could happen on the Acoustic Planet Tour is a busted string. Such is the contrast between the jam scene and the industry mainstream. Whereas the big-ticket tours have languished in the summer heat, hardened road warriors Bela Fleck, Keller Williams and the Yonder Mountain String Band (pictured) have had little trouble staying above the fray. Plenty of credit goes to faithful followings, but there's also a sense of beautiful simplicity that comes with a clean banjo line, a hot mandolin lick or layered guitar loops. If summers were made for front-porch pickin' and grinnin' with friends, it's not too hard to understand why a stage full of glam or an army of backup dancers might have a hard time keeping their summer shindigs going.