I think that it is quite unlikely we will see a new design, or maybe I should say a radically new design.
In order to support this, I point towards the rumors around the 15" MBP possibly being redesigned to reflect the MBA styling. We are seeing rumors about this in anticipation of an April release (though I think that is too early of an estimate).
Presumably, one aspect of what is fueling these rumors is sightings/reports of such hardware in the supply chains (think prototypes). We are not seeing any rumors of an iMac redesign when (as I would argue) it is more likely to be released before the new MBPs. The lack of rumors about a product that (if redesigned) would likely be undergoing very limited test productions at this point, then makes me believe we won't see a new design.
It could be that the new design is so minimal it is not worth reporting and/or it is not that noticeable, however, there would have to be some altering of current production lines to accomodate a new design which would likely lead to new rumors.
I should clarify that I am not saying there will be no redesign for sure, just that I don't think the current data supports a conclusion of expecting one.