First an observation about the title of the thread. There is no such thing as a dissonant scale. Dissonance is the result of two or more simultaneously sounding notes. And almost any scale can provide two notes that are dissonant.

Originally Posted by: Ian HandHas anyone got any idea what scale or scales Robert Fripp or the guitarist from Primus use to get those discordant solos?[/QUOTE]
What Fripp song or solo are you talking about? King Crimson early, middle, present? League of Gentlemen? Eno projects? Crafty Guitarist?

Fripp gets a lot of milage our of every scale he uses. :)

Larry from Primus usually just plays a lot of diads & triads containing flat 5ths, 2nds and flat 2nds in order to get that effect. Examples:

E|------------------------------------|
B|--------------6--------------5------|
G|-----3--------9------3-------7------|
D|-----2---------------2--------------|
A|------------------------------------|
E|--0--------0-------0------0---------|

Also as ren said:
[QUOTE=ren]It's not always about the actual scale, it's how it fits with whatever rhythm is going on underneath. It's also not always within a scale (players frequently go outside) and even intervals of popular scales sound dissonant if played together.

Exactly.