Available Colors: Solid Black, Solid White, Titanium Metallic, Midnight Silver Metallic, Obsidian Black Metallic, Deep Blue Metallic, Pearl White Multicoat, Red Multicoat, Signature Red Multicoat
Wheels: 20" or 22" silver turbines (The 20" come with Continental all-season & 22" with Pirelli high-performance)
Interior options: Black Leather, Tan Leather, Ultra White (formerly White Leather, but now possibly some durable composite? "White Luxury Seats are finished in high quality synthetic leather that resists wear and staining." )
Trim choices: Dark Ash Wood, Figured Ash Wood, Glossy Obeche Wood, Matte Obeche Wood, Carbon Fiber
Headliner: White or Black Alcantara
Base Price: $132,000 + $1,200 Destination
Vehicle Dimensions: Length 198", height 66.3" (doors closed), width 89.4", ground clearance 7.2" (via Lyon PDF service center got)
First Production Delivery (of Founders): Sept 29  at our Fremont factory (Source: Elon Tweet 2-Sep-2015 1:59pm PDT)
Let's add two screen shots that will explain most everything. What are missing are the screen shots of paint, wheels and interior. As with Signature S, the original Signature Multi-Coat Red is back. All other colors for S are available for Signature X. New Turbine design wheels are available in 20 or 22 inches. White leather returns for the Signature X as an option along with Tan or Black seat colors. Dash materials have one added, Dark Ash Wood. Notice that the price on the first screen is without the three available options. The second screen shows the price with all three options. Also notice the possible delay with the Subzero Weather Package.
So the rear accessory hitch and retracting spoiler are standard. Independently operable 3 adult second row seats will apparently be an option (although standard on the signature). I guess without that option you'll get a bench in the second row?