Just think about a winnebago for a second..... now think about cramming all that onto two wheels.  I mean, look at the size of this bedroom:  http://www.winnebagoind.com/

This is my list so far (increasing at a silly pace):

Tools / Spares
 
Spare keys
Back and front tubes
Chain links
Oil filter
Spare wire
Spark plugs
Fuses
Soft mallet
Screwdrivers - flat and philips
Leatherman
Allen keys
Bungees
Adjustable spanner
Mole grips
Spoke key
Spokes
Ratchet & sproket set
Tire compressor
Chain lube
Tool box
Locktite
Tyre levers
Tiny tiny files

 
Bike

Bike locks
Panniers
Top box
Gps mount

 
Clothing

Helmet
Gloves
Summer gloves
Cargo pants (kevlar built in!)
Jeans
Pants (thats undergarments to you yanks) x 4
Fleece
Socks x 4
walking boots
Gaiters
Thermals
Shorts
Beanie
Cap
Shades
T-shirts x 2
Rock climbing shoes
Tevas
Neck warmer 
 
Documentation

Libretta
Carnet
Passport and copies
Driving license and copies
International driving permit and copies
Bank statements
Visa
Maps
Pilots license
Flight tickets
Insurance docs
Vehicle ownership docs
 

Camping

Head torch (always a good look)
Tent
Sleeping bag
Thermarest + pungy kit
Tent pegs
Washing line
Msr multifuel expedition stove
Msr cannister
Msr titanium pots n pans
Ear plugs
Stove spares
Water carrier
Sunscreen
Toothbrush and paste
Compass
Needle and thread
First aid kit
Tarp for bike
Dental floss
Lighter
Shampoo
Soap
 
 
Misc

Notebook
Pda (for music)
Gps
Gps maps
Koss earplugs
Digital camera
Autoroute cd
Garmin gps cd
XP cd, Office cd