Wikipedia Mobile [iTunes link] brings the full boatload of Wikipedia knowledge to a nicely formatted iPhone screen. In my tests, it responded quickly, and without any glitches over Wi-Fi and the 3G network.
The only issues really are that there are already dozens of similar apps available for the iPhone. In fact, if you go to the Wikipedia site in Safari things are nicely optimized for the iPhone screen. Of course the app has a built in history button, but Safari does too.
Some of the other Wikipedia apps I've tried are Wikipanion [iTunes link], which has more features, including changing the font size, and Wikiamo [iTunes link] that allows landscape view, reading of saved material offline, and links to other languages.
I'd like to see the ability to email entries to myself or others, and the Wikipedia Mobile support page just goes to the home page of Wikipedia. That's not too helpful.
One other odd thing. When trying to install the software on a first generation iPod touch, users are reporting it fails with an error message saying a microphone is needed. Huh?
However you access it, Wikipedia is a great resource, and the Wikimedia Foundation says it will be updating the app frequently based on user requests. The current version is 0.1, so I think this app is just getting started.