What is the significance of water from Jesus' side? John 19:34 "one of the soldiers pierced Jesus' side with a spear,
  bringing a sudden flow of blood and water."


I know this is supposed to be something great as in the next verse the apostle tries to ensure us just how true this is - 


  John 19:34 The man who saw it has given testimony, and his testimony is true. He
  knows that he tells the truth, and he testifies so that you also may
  believe.


What is so significant about this? Is it symbolic of something?
Mason and Dan have both noted the literal significance of the blood/water issue. It was a medical indication that A) he was dead and B) he didn't die as the direct result of the crucifixion.

On practical thing we can note from this is that Jesus was in control of things right up to the end. We know from Scripture that nobody forced Jesus's life from him, he gave it up willingly. This is rightly applied to Jesus' willing submission to his captors, even stopping his disciples from fighting his arrest. However, we see this is true all the way to the end, when it is Jesus who surrenders himself to his Father and lets go of his own life rather than the physical reality of the cross actually taking his life.


  Luke 23:46 (ESV)
  46&nbsp; Then Jesus, calling out with a loud voice, said, “Father, into your hands I commit my spirit!” And having said this he breathed his last.


Another practical result that came out of the deal was that finding him already dead at such an early time frame, they did not go on to break his legs to induce a quick death by suffocation as would have been required otherwise.

Thus, something that was said about him in the OT that could have been understood figuratively, also literally happened in spite of the odds.


  Psalm 34:20 (ESV)
  He keeps all his bones;
  &nbsp;&nbsp;   not one of them is broken.