We regularly travel all over the world (my dx child is now 13) and fly 5-6 times a year, both domestically and internationally. We have learned through error (lots of errors!) . Now we have this down to an art.

Ditto what the people above said, with a few additions.
-NEVER pack ANY diabetic supplies in your check-in luggage. ALWAYS carry on. We usually take a separate carry-on bag with the diabetic supplies. Make sure you have a note from your doctor allowing you to carry the supplies. 90% of the time you will not need it, but every once in a while....
-ALWAYS take extra meters! We have had meter issues while travelling on more than one occassion, including my son leaving it in an airport once during a connection! (duuuh)
-If you are pumping, be sure to take not only syringes in case of pump failure, but also be sure to take LANTUS. If the pump fails you will NEED the lantus, or you will be giving shots hourly around the clock. (learned that one)
-If you are pumping and are travelling internationally, TAKE A BACK-UP PUMP. Medtronic will send you a "loaner" pump to take with you just in case your pump fails. Be sure you have all the pump settings written down, and take them with you in case you have to start the new pump. (has happened to us once)
- even though you can't take liquids through security, they WILL allow you to take juice boxes if its packed with your diabetes supplies in the carry-on.
-Be sure to play the "diabetes card". Take advantage of airline pre-boarding so you can get settled, do blood sugars, and make any corrections you need to beore the flight gets going. They always allow our entire family to pre-board. ALso, make the flight attendants aware of your child's condition. They can be very helpful if you have an problems such as a sudden low. You can also play the diabetes card if you have trouble being seated together. "My child has diabetes" usually does the trick, and seats will get shuffled around.
- We usually take several weeks' worth of EXTRA supplies, on top of what we think we will need. Better safe than sorrry. Don't forget the ketostix!
-from a hotel standpoint, make yourself familiar with ADA regulations. The hotels must provide you with a FREE refrigerator in your room (upon request). Also, for hotels with adjoining rooms (usually chain hotels, such as Marriott), sometimes they tell you you can request but they will not "guarantee" adjoining rooms. Mention the ADA and they will have to guarantee the adjoining rooms so that you can monitor your child in the night. (we have three kids and now that they are older its just too crowded to squish into one room).
-LASTLY - do not ever let the diabetes stop you from going anywhere or doing anything you want. My son has gone "extreme caving" in Belize (7 hours in the pitch dark in chest-deep water - with LOTS of precautions including a satellite phone), scuba diving in the Carribean, luging in the Alps, snowboarding in Utah, offroading in southern Mexico, exploring mines in Austria, surfing in HAwaii, fishing in Central America, and a million other things. WIth the correct precautions, they can do ANYTHING.
HAVE A GREAT TRIP!