Here's the stuff I've bought to go with or that I've used with my iPad.
Kensington 10" NetBook Sleeve - I bought this one because it was cheap. It fits the iPad perfectly. I paid $6 and on Amazon it had free Super Saver Shipping. I can slip the iPad in and then toss it into my bag and know it has at least a little cushioning. Plus it makes it less slick to hold, so i feel a little more secure carrying it under my arm.
Pogo Stylus - I wanted something to write with other than my finger. It makes it much easier to use AirSketch. It ran me $7 with free shipping from Amazon. It works pretty well, but it's small. In fact it's so small that it fell out of my bag one day and I didn't notice it was missing till I got home.
Acase stylus - More expensive ($12) than the Pogo stylus, but worth it. It's still not full pen sized, but it is bigger and easier to hold. It slides over the screen much better than the Pogo as well. I didn't realize how much friction there was with the Pogo until I tried this stylus. Something like this is a must for anyone who wants to write on the iPad. The other cool thing is you can even use it while wearing gloves.
Gorilla Pod - This is actually a really cool camera tripod, but lately I've been using it as an iPad stand. I just fold it around and it works pretty well. In addition I have it with me whenever I have my camera.
Belkin Grip360 - I just bought this. I'd been looking at it on Amazon and the price dropped from $70 to $50 so I jumped. Basically it is a hard case that incorporates a rotating hand strap on the back. I typically carry my iPad around my classroom and I've been very worried about dropping it. This will make that much less likely. Additionally you can swap out hand strap with a stand, so I'll probably leave my Gorilla Pod in my camera bag from now on. I haven't used it in my classroom yet, so I have no idea how the added weight will affect my arm/wrist after a full day.