I've used a tablet and an iPhone for a while in the cockpit. I don't find the cellular networks very useful in flight.
I far prefer my tablet computer with OziExplorer for sectionals, plus GoodReader for pdf based materials - approach plates mostly. I use GoodReader on both the tablet and the iPhone, but loading the files is a pain in the butt because of Apple's restrictions.
I get my plates from pdfplates.com - free site, run on donations only. Hope they can continue, though they'll have to convert to a fee service at some point. I get sectionals and IFR enroutes from NOAA directly for a buck or so each.
I'd love to have a iPAD, but will await the ineveitable arrival of viewers / GPS moving maps that permit easily preloading and "page sized" tiled images - as I've not found a good way to view sectional-sized images on the iPhone due to processor limitations when manipulating such large jpgs.
There are some public domain "tiling" solutions out there, but none are "easy" yet. And this is a requirement to easily load sectiona- I use GoodReader on my iPHone and even though I'm a software / computers / network professional, the content loading is a pain and doesn't need to be (hear that Apple? - disclosure: I used to work for Apple).