My husband travels frequently in and out of the US due to his job. His interview is scheduled for May 2008. We are concerned about the time between the interview to the oath and to the time he actually gets an US passport. Will there be any time that he may not be able to re-enter the US, while we are waiting on the US passport? If they keep his geencard at the interview or the oath, will he get a document so he can enter with his Mexican passport, while the US passport is being processed?

No, he will need to enter the US with a US passport. There is no document or anything else you can use. At one point a few people had managed (with some difficulty) used their Naturalization Certificate to enter, though pretty sure that no longer works with the increased border security.

You either will have to expediate the passport and wait the few weeks for that to come, or post-pone the trip until the passport comes. If not, you may attempt to postpone the oath, which then could be who knows how long until you get that again...

Actually, for land travel, the certificate of naturalization should still work, if accompanied by a driver's license or similar identity document. CBP says they'll accept it, and the state department and DHS agree.

For air travel, a US Citizen requires a US passport. There are no substitutes, and a foreign passport can't be used at a US POE by a US Citizen.

Passports won't be required for land travel until June 1, 2009; over a year from now. Congress has pushed this deadline back several times.

References:

http://travel.state.gov/travel/cbpmc/cbpmc_2223.html
http://www.dhs.gov/xtrvlsec/crossingborders/index.shtm#0
http://www.cbp.gov/xp/cgov/travel/vacation...e_in_proced.xml

If he has to travel on short notice (within 14 days), he can make an appointment at a passport office (there are 13 around the country I believe, the ones in the southern US are in Miami, New Orleans, Houston or Los Angeles) and get a US Passport the same day. There are companies that can provide this service as well, if your husband does not want to appear himself. Your husband's employer should have no problem with paying for one of these services in order to get a US Passport very fast. But it is true, once he is naturalized, he must enter the US with a US Passport only.

Thanks, lucyrich, your information is helpful.

We plan on entering Canada by sea (ferry) and coming back over land in two weeks. We'll let everyone know how it goes.