After just sitting here re-reading it all I can say is enjoy (and I hope it helps...)
I've just had something that sounds the same. I went out in my car in the morning, went home, and when I went out later on it just wouldn't start, I got headlights, stereo, everything electrical seemed to work but the car just wouldnt turn over. Spent ages under the hood, checking things. Nothing out of the ordinary, a few days later I checked the fuse box again and there was a blown fuse! (no henestly I checked, there wasn't any blown before) Replaced it and it turned over but still wouldn't start. After a couple of attempts I got a loud bang from the engine bay and see smoke, when I looked in the engine bay I see smoke coming out from the throttle body (which just turned out to be a backfire due to all the fuel). And after that I decided to get some one out to hook it up to the diagnistics, thought it might be safer for my car. He came out and hooked this thing up to the car that reads out the faults, the one I got was 26 (poor battery) so he connected some kind of jump start box to the battery and turned it over, after a couple of attempts it started up. Been fine ever since. He recons its either the battery which isnt holding charge or the alternator which isn't charging the battery in the first place.
But I have heard of a battery fault in which it run things like the stereo and lighs, but still dosn't have enough charge to start the car, so try getting a multi meter on it to see what the voltage is (or stick a battery from a good car in) solenoids are not un heard of going faulty (theres a way of telling this when you first turn the key but I cant really remember it so I wont offer my advice on that) Can you hear the petrol pump click when you first turn the key? Could be bad wireing with the ignition (this can also be checked with a multi meter) heard of that before, or the wireing for the imboliser.
Could really be a number of things, but the first things i'd check is the battery/starter motor/alternator (not forgetting fuses!!!) If in doubt get a trained person to have a look over the car, they can hook up their software to diagnose any faults.
<img src='http://img.photobucket.com/albums/v225/2ndclasscitizen/martinmkcsig.jpg' border='0' alt='user posted image' />
<span style='font-size:9pt;line-height:100%'>It's better than doing crack.</span>