Flash for sure will be essential for both a fill outside and to light the reception hall. Depending on restrictions where the ceremony is you may want a longer range lens. My favorite lens for wedding ceremonies is the 70-200 2.8 because even when the church restricts photographers to behind the last pew I can still get some close ups and the 2.8 allows me to get low light shots if flash is restricted. You'll want to see if you can get or borrow a back up camera body. When we shoot we have the 10D, D70 and usually a film camera as well.
Other essentials in our bags are:
-Extra batteries (rechargables are a good bet)
-15 minute charger if you go the rechargable route
-lens cloth
-Diffuser for the flash
-If you have film camera for a backup bring at least 4 rolls of film
Eventhough you didn't ask, I'll give some advice anyway
-Vist the locations before hand and talk with the official performing the ceremony to make sure you're on the same page
-Take practice shots to test the lighting, make sure to ask whether the lights will be on/off during the ceremony, etc
-Do a test run from house to church to reception and make sure you know the fastest way to get there
-Have a list of photos that the bride/groom want for the day
-Index cards work well as a guide to map out the order photos will be taken
-Know their timeframes, how long will you have to take photos between the ceremony and reception
-Talk to the DJ or band first thing when you get to the reception, ask the order of events and a time line for things like the dance, cake cutting, etc.
I'm sure you'll get lots of advice, and see that we all do things a little differently
