I guess they are going to get on acoustic guitars and play Tap and Folksmen tunes and talk about the movies, crack jokes etc. I imagine the Folksmen stuff will translate better to acoustic instruments than the Spinal Tap material!
Still, it sounds intriguing and is certainly a different idea. From the official Web site:
UNWIGGED & UNPLUGGED is not about funny costumes and characters, but rather three long time friends and collaborators who make each laugh and love nothing better than to find some excuse to get together and play music. The trio commented, “With this being the 25th Anniversary of the film This is Spinal Tap, we thought this would be a fun and at the same time, a little challenging, as we have never performed as ourselves. Think of the evening as three old friends jamming in your living room. As opposed to OUR living rooms; we won’t be home.”
Here is an interview with the trio where they mug it up a bit and talk about the tour:
And a YouTube video that I can't get to play. I sure hope it's funny: