Usability 
The latest version of Yahoo Mobile is a very flexible and open web portal which allows you to add virtually any sort of content, including third party content like Gmail, Hotmail, AOL Mail, Twitter, MySpace, Facebook, Friendster, YouTube, Bebo, any RSS feed and just about every Yahoo service that exists including weather, over a dozen different Yahoo News categories, stock quotes and movie showtimes.
The Twitter integration is great, definitely the prettiest mobile web interface to Twitter yet. It's a little disconcerting though that links in tweets are plain text. However, if you click a tweet it opens up the author's timeline in Twitter Mobile where you can follow the link. The extra click doesn't add any value, why not just let me follow the link in the first place?
The Gmail interface works kind of the same way. The Yahoo Mobile homepage shows you how many unread emails you have and when you click on the status it opens Gmail's own mobile web interface.
Yahoo is limits access to the new portal browsers that are able to display it well.That includes the iPhone, Android, Symbian, BlackBerry, Opera Mini, Opera Mobile 9.5 and Samsung Instinctl, older devices get the legacy version (bottom inmage). It includes Yahoo's great news and sports coverage, the web, mobile web and image search components of oneSearch, driving directions and access to your Flickr photos, Yahoo email, address book and calendar and IM.
Yahoo has had a mobile presence since 1999. At first the service was aimed at PDA's. But by early 2000 Yahoo had a WAP site which not only had news, sports, finance sections, weather, White and Yellow Pages search and driving directions, but also allowed users access to their mail accounts, calendar, address book, Yahoo messenger and Yahoo auctions.
Early on, Yahoo had the ability to sync your Yahoo calendar and address book with Outlook, Notes, ACT or Palm Desktop. This meant that with WAP your phone could give you access to your business appointments and contacts. Simultaneously with the WAP portal launch, Yahoo released Mobile Alerts, a rules based engine that would send a text message to your phone when an email arrived, a meeting was starting soon or a stock price passed a trigger amount. This combination of integrated was very advanced stuff for it's time. 1999 and 2000 were the heady days of early WAP enthusiasm. However, user acceptance of WAP failed to meet expectations as the high data costs, slow response and limited content of early WAP did not live up to the initial hype. Nonetheless, Yahoo continued to maintain and support their WAP services but did not make any major enhancements for four years.
Late in 2004, Yahoo released a completely redesigned mobile portal. The new site was WAP2, although the original WAP1 version is still available and works as well as ever. The new portal features all the content of the WAP1 version enhanced with small photos accompanying some of the news stories plus two new features Yahoo Mobile Search and a RSS reader.
In July, 2005 Yahoo launched SMS search. You text it a search string and it sends back the results in another SMS. On the mobile web side Yahoo added Local and Shopping (review) searches in 2005.
Yahoo redesigned their mobile home page again in August 2006. The new design was more attractive but no longer had accesskeys, reducing usability. Read more about that Yahoo redesign on my blog.
Yahoo's oneSearch for mobile came out in 2007. It combines results from Yahoo's Web, Mobile Web, Shopping Local, News and image searches on a single page . You can also set filters to return only one type of results, like shopping or images.