I found this blog post on Search Marketing Standard.  It points out the fact that Yahoo! still gets a big chunk (20 percent) of all searches done online.

Here's the article:

Google’s influence on the web is so powerful that we tend to forget that there is another search engine. The forefather of modern search, Yahoo! still serves around 20% of all internet search queries. This pales in comparison to Google’s close to 70% majority but is spread over a different demographic making it an intriguing prospect for certain types of web properties.

The Yahoo! homepage is laden with information, offering up news, entertainment and weather alongside its search facility. This makes it a particular favorite of female web users and some parts of the business community. As Yahoo! Panama advertisers will know, Yahoo also boasts strong conversions (often superior to Google), indicating its traffic is cash rich and prepared to buy online.

Optimizing your site for Yahoo! calls for a back to basics school of thought with many of its current requirements built around common search principles and out of favor Google yardsticks.

Meta Data: It’s commonly known that Google web bots do not heed meta data, preferring instead to focus on the meat and bones of the page they’re looking at. Meta data is open to keyword stuffing and over enthusiastic use of key phrases which perhaps accounts for Google’s decision to wipe it from its SEO checklist a while back. Yahoo! however are sticking with meta data as part of their thought process so it’s back to Meta Data 101 for those of us keen to see our Yahoo! rankings gather pace.

Yahoo! will consider good meta titles, keywords and description tags to be ones that most accurately convey the contents of the page. The title should be as succinct as possible, with no more than 5-10 words. Of this word limit, it’s essential that keywords are included. Similar rules apply for the meta keyword portion of the HTML coding. The same less is more rule applies but, when compiling meta keyword descriptions for Yahoo, the keywords of most relevance and importance should be weighted as close to the start of the tag as possible. As with keyword stuffing content, over use of keywords will be judged harshly so avoid the temptation to add too many geographical, misspells and singular/plural versions of your primary phrases.

Page Structure: Yahoo! again follows closely in Google’s footsteps but placing a greater emphasis on the basics of page structure optimization will be a bigger boost to your Yahoo! efforts that it would be on Google. Don’t be surprised to see correct use of H1 and H2 tags, Alt tags and internal links acting as search position fuel. While these small changes are somewhat of an accessibility issue and therefore play a small role in upward mobility on Google, they are much more powerful ranking tools for the marketer aiming for better Yahoo! positions.

If you’ve recently had your website professionally designed, you’d expect that most of these page structure elements are already being actioned. Don’t be surprised to find that they’re not. Google wields such power that many modern sites are built with just the one search engine in mind so some simple tweaking achieved through basic HTML knowledge can make all the difference. Sites lacking ALT tags and correct H1 and H2 titles are quickly and easily remedied. Look out for keywords in both instances, using the primary keyword in the H1 tag to increase the visibility of the phrase and useful, accurate descriptions in the Alt tags.

External Links: Link building is the bread and butter of most optimization campaigns but, due to the sheer amount of competition on Google, the focus is often on quantity and not quality. This is a big mistake if you’re optimizing a site for Yahoo! inclusion. While there’s a huge divide in opinion as to how relevant the results that Google returns are, the search engine does site relevance as a core component of its ranking considerations. The same is true and then some of Yahoo! To make sure your links are really helping your bid to move further up the Yahoo! ladder they must be as closely related to your web site’s subject matter as possible. That means seeking out sites for link backs that talk about products and services like your own, that use similar keywords, include similar meta details and use your keywords in the anchor text.