I content that you will
know at least some of the theory behind music if you play any instrument at all. Even if you're just sitting around reading sheet music or tab you can't help but pick up on the patterns of scales or the progression of chords or the intricacies of correct timing. You may not know the formal names -- like acs is talking about -- but you don't really have
to know that stuff to know what sounds good and what doesn't. On the other hand I know exactly where ZIG is coming from. I think you need to know that kind of stuff if you're wanting to write a lot of different, more complex music (at least that's what I've come to find out) but if you can slap together a couple of cool solos, throw in a few fills every now and then, and basically be competent enough to sound good to yourself and/or others isn't that what it's really all about?