ignition switch, or any number of relays...
what exactly happens when you turn the key to crank the engine? Do accessories like the stereo turn off for cranking like normal? Do you hear a very fast 'clicking' sound (not starter click from a dead battery, but an electrical click...relays often do this)?...
Someone just last week was having a similar issue with a bad alternator...I'm not sure if he figured it out yet either, but we were narrowing it down to a specific relay...
One thing, that is probably obvious...nothing happened with the alarm, right? you haven't attempted to start it without disarming or something? Some members have had some goofy electrical issues where the alarm wouldn't click off even with unlocking the door...and thus...no start...but, i'd imagine you'd be aware if that red light was blinking right in your face non-stop haha...