Welcome to cruise chat, psychocruiser!
Ah, the Mexican Riviera; good question about the safety of the region. I'm booked for 3/16/08 and have some similar concerns. Certainly, if you take any excursions, be sure the company is reputable, and I'd be cautious about taking anything more than a mile or 2 inland. As the date of our cruise gets closer, I'll be looking very closely at the political tenor of the region; check the state department's travel advisories, as well.
The good news is that it does not appear the ports are in the middle of the war zone or in the middle of the "hot" smuggling (drugs and humans) transportation routes into this country. They are on routes into the country, but not yet "hot" routes...