Leo's Wiki for Android
Leo's Wikipedia Reader for Android
A offline wikipedia reader for Android

Downloads
Android executable Install file. Copy to SD Card and run it using a file manager.
Wikipedia Reader for Android. Source code Source code of Wikipedia Reader for Android
Wiki conversion tool Wikipedia XML Dump to Reader (Android/Jinke ebook) format
Simplewiki Simple wiki is a reduced wikipedia with most important articles of the encyclopedia

Screenshoots
WIKI_ANDROID
WIKI_ANDROID
WIKI_ANDROID

How make your own wikipedia files
Download pages-articles.xml.bz2 from http://download.wikimedia.org/dewiki/20100206/ and extract the xml file using a compressor tool like 7zip.
Wikiconvert tool
uncompress this file in the same folder as the xml and type:
wikiconvert "your downloaded and extracted xml file" -owiki_prefix_like_enwiki -n
In some minutes you get some wiki_prefix_like_enwiki*.* files. Copy to the sdcard of your phone and run the wikipedia reader.
There is no problem having several wikipedias in your sdcard.