ATI said they are going to put 4 DX11 GPUs this year.
This means we can't be sure chip showed to public is RV870. It's equally possible that they are showing RV840. Bear in mind that if RV870 would still be planned to be 250mm2+ then TSMC 40nm process might not be up there yet to reliably produce such a die, therefore AMD decided to show off something they are certain they can meet demand for in case of early launch.

Just a speculation

On the other hand 180mm2+ is very close to what we had with RV670