Ok I found the answer here:
QUOTE
Nonresident alien spouse. You are considered unmarried for head of household purposes if your spouse was a nonresident alien at any time during the year and you do not choose to treat your nonresident spouse as a resident alien. However, your spouse is not a qualifying person for head of household purposes. You must have another qualifying person and meet the other tests to be eligible to file as a head of household.
So basically you can still file head-of-household (assuming that is what you've been doing in previous years and you are qualified to do so) even if he does arrive anytime in the year so long as it was not a full year I guess. I don't know why you'd elect to treat him as a resident alien for tax purposes since you'd miss out on the huge head of household deduction, unless it's a visa thing where they'd use that against you somehow.