It's considered more of a rumor and these aren't official, but it seems achievements are being leaked more and more.
There is a video in the link showing the achievements. It obviously contains a few spoilers, but for those of you who don't want to know, here's a quote.
If you don't want to spoil it, expect achievements for campaign progression, Firefight, a few collection achievements, some combat achievements, Vidmaster achievements ... in fact, they are of a similar nature to Halo 3 ... go figure!!