Usually if there is an issue with the six-month expiration it is not with immigration in the destination country but with the airline you are flying in on. A gate agent with a stickler for detail can deny you boarding.
Does this happen often? No. Does it ever happen? Yes. There was a posting about this a few weeks ago on the Lonely Planet Thorntree board from a person who had been denied boarding on a flight to Panama by the airline, due to the fact the passport expired in less than six months. A friend flying from Florida to Belize on American a few years ago was almost denied boarding but was able to talk the agent out of it, arguing that it didn't apply since he was a Canadian. The agent bought it.
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On expedited processing, many U.S. Congresspersons provide expedited passport processing as a constituent service -- this may not be quite as fast as the expedited service you pay for, but it's free (except perhaps for the cost of Fedexing.) You can call your Congressional Representative's field office and see if that's a service offered to voters. (If not, maybe it's time for a new Congressperson.) And ~$15 bucks is a heck of a lot better than $300.
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You do send in your old passport when reapplying, but you get the passport back, either deactivated electronically or with a few holes punched in it.
--Lan Sluder