This pamphlet, put out by the South Carolina Department of Health, suggests it is safe at one month. You should avoid large crowds, but the fresh air and stimulation is good for baby. You should probably not allow people to touch your baby's hands (since baby may put them in his mouth). Your greater concern in going outside is the weather - keeping baby warm/cool enough and avoiding too much direct exposure to sun.
Personally, in a non-crowded situation, I would not hesitate to take the baby for a walk as soon as I felt up to it - usually within days of birth.