Based on my polling experiences, I can virtually guarantee you that a big percentage of IRV voters are going to rank Bush top and Kerry bottom, or vice versa, even though neither is their honest opinion.
You can fantasize all you want about how that isn't necessarily always strategically best, or they wouldn't have been smart enough to think of that, or whatever. Reality is, they'll do it.
Now assuming this happens, it is goodbye to all third parties: under the rules of the IRV system, none can win, so game over.
(What if the same thing happens under range voting? No problem, under those circumstances a third party can still win easily under the rules of range voting.)
It's that simple. So if you are a third party, we hope you are listening.