Speaking only as an EC2 user, I have not seen any evidence that there's any kind of "king of the hill" bidding going on w/ spot instances. You get machines if your "bid" exceeds the current spot price AND there is available capacity. What controls the current spot price? I don't know.
I get the feeling that Amazon is experimenting with really trying to create a market here, but that there are still some very strong controls and limits with respect to how the pricing is computed. For example, most spot prices tend to max out at the reserved instance rate. I highly doubt that's a market effect. It's certainly worth bidding over the reserved, but under the normal, instance rate. I'm guessing that it's going to be difficult to create a market that is unrestricted but still preserves the quality of the user experience, in terms of rapid oscillations around different support points of bid prices, preventing anyone from keeping a server up for a coherent amount of time. You could bid-bomb DOS the market too, for example, if you were willing to spend a little money. You only need to hold the machines for an hour...