I recently stumbled across an offline Wikipedia for the iPhone and iPod Touch. I don't have either, but Shelley has an iPod touch so I installed it there. It wasn't free but it was less than $10. You buy the app from the iTunes store and then when you run it for the first time it downloads the actual Wikipedia. It's 2gb so it takes a while. (and uses up space)
I had a copy of Wikipedia on my Palm and since I drifted away from using/carrying my Palm it's one of the few things I miss.
This version doesn't have any images, but otherwise it seems pretty good. The searching isn't the greatest, for example Elbrus and Mt Elbrus didn't find anything, but Mount Elbrus did.
I'm not sure exactly why I love having an encyclopedia at my fingertips. But there's something about having so much "knowledge" in your pocket. I'm just naturally curious I guess.
Despite trying to cut down on my gadget addiction, this adds another justification for buying an iPhone or iPod Touch. I hate the monthly fees and long term contracts with the iPhone, but it's definitely a more versatile gadget.