When and how are you going?
The US government plans to start requiring passports from everyone to get back into the US as of Dec 31, 2006, if they are coming in through an airport. A year later, they'll need them if you're driving in. I can't remember when they'll need them from people arriving by boat.
If this doesn't affect you, I'd get one anyway. Only 10% (or less) of Americans have a passport. When all the people who go to Mexico every year suddenly realize they need a passport now, there's going to be a stampede for the passport office, and who knows how long it'll take to get one? Better to get one now and avoid the rush.
They have delayed this a couple of times already out of fear of that stampede, so it's possible it'll get delayed again, but I wouldn't rely on that.
