IGN recently spoke to Bungie's Lars Bakken. He stated that Halo: Reach would be their last in the series.
"After Reach that's it for us. We're already working on a new IP that we can't talk about yet, I don't know when we'll be talking about it - when we're ready I suppose."
When questioned about the new IP, Lars was very vague:
"If you look at Bungie's past we've done a lot of different styles - first person to third person to RTS - so it could be anything."
'Halo 3: ODST' is due in Autumn, while 'Halo: Reach' is expected around the same time next year.