Did the factory tour with my family last week. I recommend it if you are in the San Francisco area, the factory is a pleasant 45 minute drive from SF and the tour lasts around an hour.
It starts with a chance to view the Model S beta prototypes and the Roadster that featured in Iron Man's garage and continues with a tram ride round the factory. There's a stop midway to walk up on to an overhead gantry and look down on the robots welding up the main body pressings in to complete body units prior to painting.
Due to the factory layout, it doesn't quite follow the production process in sequence. You get to see:
> The press shop and the correction/hand finishing area
> Robot assembly and welding of the pressed components in to a body in white
> Dashboard, drive train and suspension sub-assembly
> Lots of battery packs, front and rear drive units and vehicle bodies ready for assembly
> The conveyor assembly lines through to final finishing and inspection
There were some items I was hoping to see which did not form part of the tour:
> The paint shop
> Battery pack assembly
> Drive motor assembly
> The pano roof fitting process
> The indoor test track
For these, watch the Mega Factories episode on YouTube
Pictures are not allowed on the tour, so I can only tell you that we saw two X engineering prototypes which were being used for assembly line training. These weren't beautiful showroom ready cars, they were rough as a bear engineering mules. The new X production line is being commissioned at the moment.
It looked to me like the new line was just the robot line for Body in White. There didn't seem to be a separate final assembly line for X. My guess, and that is all it is at best, is that the X bodies will join up with S in the paint queue and then go down the same final assembly line as S. The only information we were given is that X shares 60% of it's components with S. There are hundreds and hundreds of pages elsewhere on the forum that cover the X before launch in more detail. I do not recommend going right through them from the first page unless you have a reservation, especially if you are prone to suffering from suicidal tendencies!
A couple of people on my tour went on to collect their new cars afterwards. The factory handover area is outdoors under a canvas awning which was rather nice, this is California after all.
Worth the trip if you are in the area, I fixed up in advance through my service advisor in the UK.