Active content is a fancy way of saying that the Kindle will be able to run applications. Amazon has announced that in about a month it will release the Kindle Development Kit. There are a few restrictions mainly related to app size and the amount of traffic it will use on the Kindle's built-in wireless system.
Where I see this being useful is for interactive rule books and aids. Most of the aids needed at the RPG table are not that demanding in terms of graphics so the low refresh and greyscale only limitations of the eInk screens that the Kindle uses shouldn't be a limitation.
One compelling feature of this setup is long battery life of a Kindle especially if you don't have the wireless turned on. It book like form factor perhaps makes it a tad easier to use then a netbook or a laptop. The screen is way larger than a cellphone so that should work in it's favor over a iPhone or similar tech. Also it will be interesting to see how works with the instant-on feature of Kindle. Basically when you shut off the kindle and turn it back on you are back at where were last reading.
I may try to do something with this in between writing projects.