Here's a graph showing the Iowa Electronic Market Democratic nominee prices for the main contenders:
For the sake of clarity, I left off non-candidate Hillary Clinton (current price around $0.08), as well as the Rest-Of-Field option, which most notably includes Sen. John Edwards. At present ROF is running neck-and-neck with Joe Lieberman, at around $0.035.
Since the winning contract will ultimately be valued at $1.00 and all the others at $0.00, the prices at any given moment can be considered to represent the participants collective judgement as to the relative probabilities of the various outcome. That is, they are currently giving Dean a 44.5% chance of winning the nomination, and Clark a 20% chance.
The chart begins on 10/21 because that was the first day IEM offered a separate contract for Wesley Clark. There was no trading on 10/30 or 10/31 (server problems), and only a very nominal amount on 11/1, so I omitted the prices for that day.
I decided to post this graph because the one offered on the IEM site is fairly of such poor quality - it's almost impossible to make any sense of. I hope the folks here find it useful.