Depends. If its set in a location that I'm not particularly interested in (say the Commonwealth) I'll be excited but not overly so. I'll be more skeptical.
If however, its set in the Midwest and we see a MWBOS soldier in the teaser trailer, http://www.youtube.com/watch?v=ykpjFjsEdl4&feature=related
Also this ^ What I was getting at by details.
If there is a MWBoS soldier or symbol in the tailer, I would hit the roof with joy. If there is mention of Barnaky or Mutant Liberation Army.. I'll drive down to Bethesda HQ and Bear Hug the Devs. Thats if I can get past security guy at the front desk :fallout: