Pretty sure "Halo" is done.. Bungies said it a lot.. They said halo 3 was the conclusion of the halo story arc blah blah blah... To me that doesn't mean the end of the series...
I mean let's face it, Halo refers to forerunner ring worlds and the main threat for the last three games, now that threat has potentially been ended (hopefully the destruction of the ark has rendered the remaining ones disabled) the Halo ringwords are really not important anymore..
My view is, the games are too successful, chief will be back, it just won't be called "halo".. It'll likely have to do with that planet at the end of halo 3.. Essentially I reckon they are going to use Master Chiefs popularity to launch their next series of first person shooters which will take place far away from our own galaxy and will be more or less a clean slate for them to start a new story, killing the repetitiveness of halo (which to be honest, I never had a problem with, I'd play 10 more halos) while still keeping chief and doing something fresh..
So yeah, I figure "halo" is over, but the story isnt.