ASoS will be two seasons, with Season 3 ending with:
I suspect that Season 4 will be the second half of ASoS but bringing some elements from early AFFC/ADWD in as well. Maybe they could fit the rest of the salient points of AFFC and ADWD into just one season:
And they've said they're already thinking on how to handle ADWD, with them definitely going to follow Jorah's storyline in ADWD apparently.
But that would be a tough call.
Several recent interviews have suggested that the producers are thinking of eight seasons of ten episodes each as the ideal length for the whole series. From that I think that we can expect a significant deviation from the books, since I suspect Books 6 and 7 will have to be in the ASoS/ADWD size bracket to fit everything in, and that's assuming the series will not expand again (which I certainly wouldn't rule out). HBO certainly won't be turning ASoS, ADWD, TWoW and ADoS all into two seasons each, so that means a fair amount of trimming of the plot will take place.