^Yeah, pretty much. The mobile 4870 is actually a full blown desktop chip, but they had to underclock it due to heat issues. So yes, the mobility 4870 will perform like a desktop 4850.
On the NVidia side, the GTX280M is supposed to be the competitor, but it is once again just a refresh and isn't actually based off the GT200 core. Not really sure where it will stand.