Read your NEC Articles, boys... it's all there if you look hard enough [Linked Image] <smart aleck>

(Courtesy of the '96 NEC)

ART 320 - OPEN WIRING ON INSULATORS - Open wiring on insulators is aa exposed wiring method using cleats, knobs, tubes, and flexible tubing for the protection and support of single insulated conductors run in or on buildings, and not concealed by the building structure.

ART 324 - CONCEALED KNOB-AND-TUBE WIRING - Concealed knob-and-tube wiring - a wiring method using cleats, knobs, tubes, and flexible tubing for the protection and support of single insulated conductors

As far as multiple NM entries?

Read the Exception. It's plainly allowed.