Question: My passport expires while I am working in the US. I currently have a TN visa. I have sent the documents away by mail to get a new passport. Will this affect my status? (The passport won't be renewed in time)
Answer: The answer to your question depends on how you plan to renew your TN. Different rules apply depending on whether you obtain your TN through the USCIS service center, at a land border (POE), or at an airport pre-flight inspection (PFI) office.
Generally, a foreign national needs to maintain the validity of his or her passport throughout their stay in the U.S. If filing through the USCIS Service Center, then technically a passport should be valid at the time of extending status. See 8 CFR § 214.1 (a) (3). There is some ambiguity right now as to the extent this provision applies to Canadian citizens. Currently, Canadians do not need a passport to enter the U.S. by land or sea. But a passport is required if entering the U.S. by air. So it is not clear whether the failure to have a new passport in hand when you file to renew your TN will substantially prejudice your TN application. There is a slight risk that USCIS may ask to see a valid passport (the I-129 requests the validity dates of your passport) and issue an RFE, which would request to see a copy of the passport within a certain, limited time frame. Obviously, this could pose a problem for you.
You can still apply for your TN at a land border without a valid passport. You may also present alternative evidence of your Canadian citizenship (birth certificate). If you do not have a valid passport in hand, you will not be able to re-enter the country by applying for a TN at an airport PFI since a passport is now required for air travel to the U.S. The clearest route to avoid the passport requirement is to apply for the TN at a land border port-of-entry with evidence of your Canadian citizenship. Remember that this option may only be available until January 2008. All other scenarios may require the presentation or submission of a valid Canadian passport. I believe there is an option to expedite the processing of a Canadian passport. Requesting this service may provide you with some added assurance.