I prefer to use natural light when possible, or the light available on the show floor, which is often very good and bright. A high-power flash like the 580EX will help when we get advanced looks at products in hotel rooms or conference rooms. A mini LumiQuest softbox that folds up fits nicely on the head of the flash, and provides even, diffused lighting.
I also carry a SureFire G2X Tactical Flashlight
that puts out a powerful 200 lumen LED light.