i actually find that theory being a big deal in jazz is a fairly common misconception. In the world of musical acedemia, jazz educators try to follow the Jamie Abersold school of thought when teaching, saying 'only play on this chord this scale', etc. But along with what zyko and Snappleman were saying, i find that theory is a supplement to jazz and not much more. I learned to improvise not by learning modes and studying ii-V-Is, but rather by listening to jazz and playing along, deciding what i thought sounded good or not. The result of this was me noticing patterns in my playing, and finding that I could define these patterns in words. In short, if you want me to play a "blues scale", I might say "well, 'the' blues scale, or my blues scale?"
in short--jazz is more about playing stuff that sounds good than understanding why it sounds good.