WikiMobs is the free encyclopedia optimized for modern mobile phones. Totally web-based you don’t have to download nothing, just access wikimobs.com from your mobile browser then enter the keyword you are looking for. WikiMobs is optimized to display limited pages to limit bandwidth usage, you’ll see on top of pages the content displayed in sections and you can navigate to the section you want to read more about the topic you are looking for.
I have noticed many services providing this service, but WikiMobs is in my opinion the best web-based Wikipedia for mobile phones. The service is very new at its very early stages and currently at the version 0.5 beta. There will be certainly more features coming in the future, let me know if you have any suggestion to optimize the service and make Wikipedia access more easier from your mobile. Should I mention that it’s written in PHP ?