COD4 got the four new maps with patch v1.6 in June 2008. COD4 was released mid November 2007. So 6-7 months after release.
I seem to remember that IW/Activision wanted to charge PC players for the new maps, but Nvidia stepped in to sponsor it and it was released for free with the patch.
Given that and what we've seen with MW2, I think there's almost no chance of any new maps or content being free.
That said, I like MW2 overall and would be open to paying $5-10 on new maps and other stuff, ONLY if they support MW2 otherwise with patches. There are still hacks going on and I think part of it is pirates being able to play online so no problem if they get banned. Those things need to be fixed immediately. In addition, minor improvements to balance and fix issues should be part of patches.
They do that and I'd be open to paying for DLC if it offers enough.