GroovinGourmet is on the right track -- I-10 between El Paso and San Antonio is a vast culinary wasteland, really, but if you're willing to make some detours, the best barbecue awaits. If you get off 10 at Junction, you could hit Cooper's in Mason and/or Llano (same name/family, but different places, with the Llano one being probably my favorite barbecue in the state).
The Luling/Lockhart excursion mentioned above would also be worthwhile. My favorites in the four restaurants mentioned differ from GroovinGourmet, but maybe that's because he hasn't been to Kreuz yet :) But therein lies some of the fun of Central Texas barbecue: the general standard is high, and there are a number of amazing places, which gives you plenty of excuses to try them all, then argue about it so you have to go back and try them again, creating a virtuous cycle.
In any case, this map I've made of notable barbecue joints in the state might come in useful for you: http://blogbecue.com/?page_id=10