Travel Passport Requirements tighten up on September 30 when you can no longer use your *application* in lieu of a Passport for travel in Western Hemisphere. This passport application exception was instituted earlier this year due to the huge passport application backlog that was created when the US decided to require passports for travel to Canada and Mexico. In the past US citizens could visit Mexico and Canada without any paperwork.
For all details check the USA Passport website
To avoid potential delays and problems it is advisable to have a current passport even if you are not currently planning any international trips but might possibly travel internationally over the next few years.