According to the latest comScore numbers, Microsoft sites got around 3.7 billion searches in October. While that's only 19.5 percent of the search market share, the numbers are high enough to suggest that Bing should not be ignored - especially if your site targets an older or wealthier audience.
It's important to remember that optimizing for Bing is similar to optimizing for Google in many ways. Rule number one for both search engines seems to be high-quality content. A new post on the Bing blog details exactly what they're looking for.