No doubt you've heard about summer conditions there—highs around 120 or so, wind, blowing sand and dust. And you're right, shorts won't be appropriate for any number of reasons, so long trousers are de rigueur both for leg protection and to provide more places to put stuff.
Fortunately, several long pants qualify as good candidates. Travelsmith's Packable Explorer's Cargo Pants ($55, www.travelsmith.com) would work well—they're very light, with tough nylon material that's treated for wicking and quick drying. Not covered with pockets, but they have enough for quite a few odds and ends. And they have belt loops. I think you'd also like L.L. Bean's Tropicwear Pants ($49, www.llbean.com), made of a nylon/polyester blend that wears well and dries quickly. These pants do have big pockets on each thigh, so you can really jam them with photo accessories, lunch, and maybe a super-compact laptop computer. They come with a nylon belt that can be swapped out for another.
You also could "go native," so to speak, and wear some military- surplus khaki pants. These wouldn't be a bad choice, really—you can purchase tough cotton six-pocket pants for about $24 at sites such as www.armysurplusworld.com.
Anyway, hope that helps—and good luck. Send us some photos (or let us know where you're publishing!).