In my experience, I've rarely had reason to not have my camera with me while at Disney.
If you are primarily going as a big time ride enthusiast, it could very well get in the way.
I have made due on all but a ride such as Splash Mountain. A few rules of thumb would be these.
Do NOT assume that you can get by all day with only a strap around your shoulder. You need to have a small protective bag if you intend on riding anything that bumps ( i.e. test track, space mountain, tot etc. ).
Something along the lines of a LowePro Toploader 75 AW would be EXTREMELY helpful. It's one of the bags I bring with me depending on how much I intend to shoot that day and selection of lenses I would be bringing. It is a fairly large bag that allows you to keep the camera at your side with access to it from the top. This means you can "holster" the camera in the bag as opposed to having it simply hanging exposed at your side, so no worries about bumps against turnstiles, other visitors etc. Yet at the same time you can get to it quickly without having to worry about other items falling out while the bag is open as you might experience with some other bags ( backpack types ).
The AW stands for all weather, and this is important. As rains can come more quickly than you can find shelter at times, and as it has a little built in raincover which you can be put in place in less than a minute you can feel a little more confident that your investment isn't about to be ruined. In lieu of one of the AW bags, make sure you have some sort of plastic bag that you can put over the camera bag ( or camera ) with the opening facing down.
Another suggestion is to take a little duct tape and with the bag opening facing down, cut two holes as small as you can for the bag straps or camera straps to fit through ( assuming of course that the straps have some sort of release mechanism ) and put the duct tape on around the slots you make for the straps so they do not open further when in use.
The camera I have toted around was the Nikon D2h, which is a lot larger and heavier than the D90, so you will not likely suffer the fatigue of carrying all day as I have at times.
If you are less inclined to be riding and more of a person that takes in what is around you and focuses on the slower moving rides ( jungle cruise, train, shows ) you might be interested in a backpack more. Again my choice is a lowepro which I've packed multiple lenses, a flash and external battery and body as well as autograph book and other things for my daughter. It does not allow for the camera to be accessed for quick use though as the toploader does.
There are smaller cases available and I'm not pushing Lowepro on you, I just have found in my experience they hold up real well, are right for my needs and comfortable. But any protection is better than just keeping the camera on a strap with nothing. I don't care how careful you THINK you are, sooner or later someone or something will bump into your camera and while it may not break it, it isn't worth the risk.
What I tend to use while carrying the camera around is called the Zing grip, which is a neoprene wrap that connects to the strap fitting and the tripod threading on the bottom of the camera. It allows you to hold the camera in your one hand and release your grip without the body falling from your hands. It's suprising how comfortable it is to hold the camera in your hand as when you don't have to actually grip it at all times.
To me, I will sacrifice convenience of a point and shoot over the precision an SLR gives me.
If you plan on any meet and greets, keep in mind that no point and shoot ( or built in flash ) will be as good as an external flash unit that you might use with the SLR. Even outdoors I'll use the flash for character greetings to fill in the shadows... something that is very very hard to do with point and shoots or the pop up flashes.
I've also found that people have a little more "respect" for someone taking photo's with an SLR. I think we can agree that 90% of what makes a great photo is the photographer and the subject... not the camera... but be it as it may, a lot of people are not concerned at all for the common place. Point and shoots ( and more and more pictures being taken with phones ) are the norm and passersby seem to be a little more oblivious regarding them, and when they do see an SLR seem to give you a little more room.
As someone else pointed out, you just paid a boatload of money for that thing... why wouldn't you want to use it.
Make sure the batteries are charged. They last a long long time nowadays, but if you are using the monitor a lot, make sure you have at least 50% otherwise charge it.
Carry more than 1 card. I don't care if you have the largest card in the world... sometimes ( rarely ) poop happens and a card can go bad. Nothing makes you go batty like having a $1000. camera and having a card go bad with no backup.
Go out and shoot shoot shoot and shoot in different modes with that new camera BEFORE you get to Disney so you can see how it meters... it should work fine in auto ( if that is what you want to use ) but you want to know how to change the autofocus modes or choose aperture or shutter priority modes as well.... you don't want to be experimenting with your new camera while you are at Disney.
Best of luck... and as you can imagine keep your eye on the camera at all times.