There are various estimates of how candidates are doing in the race, for example, here, here and here. However, the consensus seems to be that the top four on the first ballot will be:
While Ignatieff will have some distance between himself and the pack the other three will be bunched.
The crucial ballot will be the second. I have heard enough anecdotally to say to me that it is possible that Ignatieff's support could then experience a sharp meltdown. It may not happen, but if it does, he would be forced out early, setting off a mad scramble for his delegates. This is what makes it difficult to forecast. If he does hang in and make it to the final ballot, it seems likely, although by no means certain, that he will squaring off against Rae.
The other scenario opens up possibilities for both Dion and Kennedy (who has been largely discounted because of his weak French). I think Dion has the "everyone's second choice" advantage here, but a scenario where the leader collapses is unprecedented, and therefore intrinsically difficult to predict. The only similarity at all that comes to mind is the 1983 PC Convention, and there, while Clark's support dipped, most of his delegates remained doggedly loyal to the end.