I recently changed jobs and find myself traveling by air significantly more often than before. I'd already developed (what I think is) a pretty good commuter kit for public transit with some great input in this thread: http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=237263 . I had to strip it down for TSA-compliance, weight and bulk to fly with it, so here's what I have with me now:

• Bottle of water, purchased locally
• snack bars
• spare CR123A battery in a plastic wrapper, suitable for my primary EDC flashlight (which is a 4Sevens Quark 123 R5)
• 4Sevens Prism kit to turn the Quark into a headlamp
• iTP A3 EOS Upgrade 1xAAA flashlight on a neck lanyard
• mini-Bic
• A couple of large paperclips
• 4Sevens titanium key ring whistle
• A small amount of duct tape
• Sawyer sunblock with bug repellant spray
• bandanna
• Heatsheets 2 person emergency blanket
• rain shell
• Countycommm "ETFR" small radio with earbuds
• Leatherman Wave (pre-2004) (in checked bag, moved to laptop bag aft flight, left at home when not checking bags on shorter trips)
• Preon 2 S3 Ti
• small FAK including Patrol Officer's Pocket Trauma Kit
• sync/charge cables and AC power cube for my iPhone and iPad
• 10 Ah lithium polymer battery with cables to charge my iThings or many other devices
• earplugs
• facial tissue
• small Leatherman multi without knife blade, marked as TSA-compliant and successfully carried through airport security many times
• appropriate outerwear, including boots when I can bring them
• spare contact lenses, glasses
• international outlet adapters if appropriate

All kits are a compromise. I've compromised here rather more than I might have liked, but for less than my coworkers traveling with me -- I've been able to help with many small travel challenges as a consequence.