Pierre Gosselin (08:35:38) :Gore said nothing of the sort, first of all. I watched the movie and have the book, no time frame for the 20 foot rise was given, but it is right in the long run. (I have, fwiw, mildly criticized Gore's omission of a time frame.)
Rahmstorf also didn't say 1.2 meters by 2100, he said between .5 and 1.2 meters by 2100. Gosselin's proposed bet "against" Rahmstorf's position actually works out to be .6 meters, so Gosselin would win even if Rahmstorf had been proven right. Gosselin's bet is even worse though, because the predictions assume that the rise would accelerate later in the century with increased warming, while Gosselin simply allocates the entire warming proportionately, an equal amount each year.
So what's a fair bet? From the viewpoint of those who are under the impression that temperatures have fallen over the last 11 years, I would guess they anticipate random change in sea level over the next five years. From a position based in reality, which is this:
I'd expect the current rate of 3.1mm/yr to continue (not much acceleration should be noticeable in just five years). Half way between delusion and reality is 1.55 mm/yr for five years, and that should be a fair bet. Of course, temperatures haven't really fallen in the last 11 years, but that's what the other side wants to argue.
I could lose this bet simply due to random noise, but it's unlikely. I'll add it to my list of proposed bets, and we'll see if Gosselin or anyone else is interested.
For some more info on sea level rise, see Deltoid.
UPDATE: edited for politeness.