the new yorker has quite a nice introductory article on the use of laptops as performance instruments.
the journalist describes the approach a few different acts use including radiohead, battles, bjork amongst others.

here's a vid of battles live on jools...

johnny greenwood using maxmsp

a short little video of radiohead's johnny greenwood being all "crazy" and using maxmsp to process his guitar playing in a live performance.
It sounds like the software is sampling the signal from his guitar and chopping it into sections and then jumping around within these samples and repeating some of them to produce a kind of stuttering effect. Its obviously kept in time with the drums (drummer may be playing to a click track) to ensure that the loops and slices are in time.

The principles of the patch are not that complex, but to create a patch that can be used easily and reliably takes quite a bit of thought.

Interview with Phil Durrant on his use of Reaktor and laptops for performance

On the Kore spinoff pages of Create Digital Music there is an interesting interview with one of the most prolific people on the Reaktor user forum - Phil Durrant (aka "sowari"). In the interview he discusses his approaches to building within Reaktor, his use of the laptop in a performance context and some of his influences.
He makes a pretty strong case for trying to get a smuch interactivity out of the standard laptop (QWERTY keys and trackpad) as possible.

Below is a video of Phil's Trio Sowari.

TRIO SOWARI
by Ornitoto