Overture is beta testing CTR as an influencing factor on bid position. They diverge from the Google model, in that they are not using Max CPC and CTR to create a rank and an actual CPC. They are using your actual bid as the price they charge you but move you up if you have a high CTR.
Hope this model works - but something tells me if they find they are getting better click numbers this way it will roll out like a variation of Google.
Hope not I have too many knocks against the Overture system now as it is.
Good luck is all I can say and I will post how it has worked after I play with it for a month or so.