Awesome camera! Probably trade off my D-90 for a 3100 in the future.
Update 10-5-10
CORRECTION I WAS CONFUSED! I was thinking from memory 7100 which is really 7000 and not 3100 either so substitute 7000 in my comments.
To answer your question let me say I have over 30 years of SLR's under my belt. I have done this and that and gone through a number of pieces of equipment. I consider the equipment that I have now to be in my possession because it is needed or required. I do some weddings as well.
Here is my list that has been perfected over the years.
Camera and SD cards
Wrist strap connected to the right side strap loop. (Dropping is bad news-I KNOW!) I don't use neck straps.
Extra camera battery and flash batteries (below)
12-200mm range of lenses. After 200mm things get less desirable for me. I used to think tele was the cats meow. Over the years my best shots were often obtained with wide angle thus my developed ho-hum attitude over 200mm. To cover this range I have and recommend
Tokina 12-24mm
Digital Wide Zooms
One or two lenses to cover 18-200mm range. I have the D90 kit 18-105 VR and an Nikon 18-200mm VR
I also have a Nikon 70-300mm G lens but will probably sell it. I bought it for my kid to take to Costa Rica.
UV filters for lens protection I don't buy the image degradation stuff for everyday not for sale stuff
Dirty lens article
Polarizer filter results in killer improvement if the conditions are polarized. I have step up rings so all my lenses can use my 82mm polarizer as well as dedicated polarizers.
SB-400 flash unless you really need something that the SB-400 don't do. At 12mm and even 18mm with my 18-200mm lens the on board flash will cause a lens shadow. So this flash is required for that as well as the bounce flash capability.
ND filters ocassionally. I use the Cokin types and just hand hold them in front of the lens.
Cheap tripod. Light and small. My favorite is a vintage brass
BILORA Biloret 2037 Compact Tripodwith or without the top slots that allow it to sit splat even with table top. Unless you have a niche that requires the often recommended big bucks pods. I just have an old heavy and very tall tripod I bought at Goodwill for when I need one to use while standing. I want it so I don't have to stoop. Otherwise my travel pod is fine for the occasional tripod need. Besides a travel pod is likely to be carried and a heavy one likely not.
Cheap Monopod from Walmart. You can set the camera timer and hoist the tripod or monopod over crowds to get shots. Or turn the camera upside down and get floor level shoots. (one poster was shooting from elevated platform for shooting bands. Mono pod down below the elevated platform) You can also use it as a walking stick.
A gym type bag to store stuff at home.
A converted lunch bag for my camera in the top pouch and the extras in the pocket underneath. What ever light and carry-able bag you can come up with and live with. I have a closet full of Lowepro top loader type camera bags that I don't use. I like my converted lunch bag the best.
Photoshop elements 5.0 I have the full blown Photoshop and others but find I always go to Elements. It is all I need 99.9% of the time. I say 100% of photos will see dramatic improvement with post processing. In other words it is not optional.
I don't use lens hoods 99.9% of the time.
That is about it. Not counting all the junk in my closet camera bag that I don't use.