Really nice open source Android app for offline Wikipedia access: Aard Dictionary for Android.

The data files for the English and German Wikipedia are a few Gigabytes, but it's worth it. Worked during the SOPA Wikipedia blackout, too. :-)

Download available on:

• Android Market
• F-Droid Open Source repository
• From the project page (Click the "Download" button.)

Getting Wikipedia onto the phone

The files take a while to copy, so this can be a small challenge over a flaky Wifi connection. I recommend using a cable instead. Here are your options:

• Your computer recognizes the device and you can access it directly. You can just copy the files over. It that works for you, do it.
• Use the Android debugger adb. The tool comes with the Android developer tools and has a facility to copy files to the device once USB debugging is enabled. The command is adb push localfile remotepath.
• Start a tiny web server: Put all files in one directory, change into it, then start up a small webserver with python -m SimpleHTTPServer. You can then download the files by directing your phone's browser to the machine where the webserver is running (port 8000).