We will not be seeing anymore updates out of Infinity Ward for CoD4. Meaning, no new playlists and no new maps. This is pretty much certain as fourzerotwo himself strongly suggested this.
They have already moved onto their next project (CoD6). It is not like most have not known this for a while though, we were only given one map pack, which was not that great to say the least. We stopped playing CoD4 anyways at the beginning of October 2008.
Source: JoyStiq