Wikipedia has a version of the site that is optimized for mobile devices, which was launched in June 2007 as a WAP site, but now is located at mobile.wikipedia.org. The mobile site was developed using Hawhaw , a php library for implementing mobile applications. For some time, there have been unofficial mobile Wikipedia sites, including wapedia.mobi which has been around since 2004.
The official mobile version of Wikipedia brings up a simple, quick-loading page, which features a search box front and center. Below the search box, there is a link to a settings page and an "About Wikipedia" page.
The settings allow you to choose a language. Currently, there are 14 languages available on the mobile site, including Arabic, Austro-Bavarian, Czech, German, Greek, Spanish, French, Hungarian, Ripuarian, Low German, Dutch, Polish, Portuguese, and Serbian. Some popular languages, including Japanese, Italian, and Russian, are not yet included. Once volunteers provide localisation for other languages, they can be enabled on the mobile site. 
The other setting option is "Spoken Wikipedia", though once you go to that page, you see it is "Not available". It is not clear what the spoken feature will entail, but there are some Wikipedia articles with audio versions, recorded by Wikipedians.
The search feature on Mobile Wikipedia works similarly to the "Go" button on the regular Wikipedia site. If there is a likely match, the mobile Wikipedia site will take you directly to the article (e.g. "Barack Obama"). If there are multiple possibilities for your search term, you may be taken to a disambiguation page (e.g. "Georgia") or a list of results. Also, only article pages are included in the search. Thus, pages useful to Wikipedia editors, such as the administrators noticeboard, are not available.
One issue with the mobile version is that the disambiguation links you see at the top of some articles when looking at the regular version of Wikipedia do not appear on the mobile version. So, if you searched "Washington", for the city, it brings up the page for Washington State, with no disambiguation link at the top. Another usability issue with the mobile site is the searching. On the regular Wikipedia site, when you start to type something in the search box, a list of possibilities pops up and helps fill in the rest of the search term. This is not available on the mobile version, so you need to type the entire search term.
Once you are on an article page, you see just the lead section of the article (above the table of contents) or possibly more, but still a small portion of the article. Loading just this portion makes the page loading much faster, and there are no images. The Barack Obama article, for example, is broken into 30 sections. Then there are links to "Continue..." to the next section of the article, and a "Contents" link to see the table of contents and skip to another section of the article. Other differences include no templates or infoboxes, no tabs (e.g. history, and talk page) and no editing.
Though there is no history tab, there is a link on each page to a "Copyrights" page, which makes available the GFDL license (a requirement for Wikipedia content), a "Source" link (takes you to the same article on the regular Wikipedia site), and a "History" link, which provides attribution to the Wikipedia editors who authored the article (also a requirement of GFDL).
In addition to the mobile site, there are already a number of iPhone apps for browsing Wikipedia, including some available for free. According to Wikimedia Foundation Advisory Board chair Angela Beesley Starling, speaking at Australia's Open Source conference linux.conf.au, the Wikimedia Foundation has plans in the works for a new mobile platform.