An interesting Windows Mobile app, which I ran across via this blog post
WikiPock for Windows Mobile is the ultimate software putting all the information you need at your fingertips. Want to look up for an acronym while you're walking down the street? Want to pull up last year's academy awards results while you're out with friends? WikiPock lets you acquire and share knowledge anywhere you are, even without internet connection. The world's biggest encyclopedia is in your Windows Mobile phone!
See the product page for more details. And how do they accomplish this magic (the part about not needing an Internet connection, or even a wireless data plan)? They simply cache a huge chunk of “the wiki encyclopedia” (perhaps a lawyer told them they can’t say “Wikipedia” in all contexts, but it’s explicit on the FAQ page; the vendor also donates 10% of its net revenue to the Wikimedia Foundation) on a microSD card on your phone. For best performance, they recommend an 8GB card.
An interesting sign of the price/performance/etc. times: the app, preloaded on an 8GB card (i.e., delivered on a new 8 GB card), starts at around $25.