khuon

One thing no one has mentioned so far is to make sure you know some basic skills. You can find some on the website but oftentimes it's much easier and more effective to have someone show you. See if your LBS has some classes or if you're buying your bike from them, ask them to quickly go through a brief introduction to bicycle mechanics and maintenance. You should be able to do things like change a tyre, adjust your saddle, inspect the bike for damage and general upkeep.

Oh and... you might want to add a few other things to your list:

• sunglasses for eye protection
• sunscreen/sunblock is also a good idea depending on conditions
• minitool such as the Topeak Alien
• mini first-aid kit

Things I carry with me when roadbiking:

• One or two (depending on length of ride) 24oz. water bottles
• SKS Airgun - CO2 inflation system with spare cartridge
  
  
• Spare tube strapped under saddlepack
  
• Spare spokes shoved into seatpost
  
• Topeak Micro Wedge Saddlepack with
  • Park VP-1 patchkit
    
  • Presta-Schrader valve adapter
  • Spare links including spare removable ConneX-Link for my chain
    
  • Small bottle of DuMonde Tech Lite
    
  • Topeak Alien minitool
    
  • Special Mavic spoke tools because I have non-standard spokes
    
• In my bike jersey pockets
  • mini-wallet with ID, credit card, debit card and small amount of cash
  • FA-112 First-Aid Kit
    
  • Kryptonite Gear Gripper lightweight retractable cord lock
    
  • Minolta DiMage X digital camera with spare battery
    
  • Speedplay Coffee Shop Caps cleat covers for my clipless pedal cleats on my shoes
    
  • A couple of plastic Ziploc bags to seal things against rain
  • Small bottle of sunscreen (SPF48)
  • Contact case
  • Rewetting drops for eyes
  • Rx insert (in microfiber sleeve for wiping off lens) for sunglasses in case I need to ditch my contacts
  • Clear lenses for sunglasses in case lighting conditions change
  • Arm warmers/leg warmers or lightweight Tyvek jacket (folded in on itself so it's very compact) depending on weather conditions
  • small mobile phone (Nokia 8260)
  • A couple of energy bars
  • compact map (if I don't know the area well) or route-map (organised ride)

Curiously enough, even with all this stuff crammed into my pockets, I still have quite a bit of room for other small things. If I think I'll really be far away from civilisation or riding where water stops aren't plentiful then I'll drag along a CamelBak Lobo and reapportion everything appropriately. I've however been able to get away with 100-120 miles without one so far.

But on really long unsupported road rides (>100mi) and on all mountain bike rides, I'll use a CamelBak MULE which will contain the following additions to the above list:

• 100 oz. hydration bladder
• Topeak minipump (with patchkit in handle)
• additional spare tubes
• two spare 9V batteries for my shocks (K2 SmartShocks - MTB only)
• mini-tripod for digital camera
• replacement/spare derailleur hanger
• spare seatpost clamp
• a couple of zip-ties
• small roll of electrical tape
• small roll of toilet paper in plastic ziploc bag

And of course if I think I'll be riding into the night, I'll have the lights and blinkers along too.