From the blog post on the Windows Live Search team's blog entitled Three new features in Live Search Images we learn

We know you love searching for people with our image search engine—we’ve worked hard to make it easy and fun. Now we’ve made it even better. Today we shipped three new features that help you find faces, portraits and black and white images. Try these links to see them in action: 

1. without the new features – jimi hendrix
2. with new face filtering on – jimi hendrix filter:face
3. with new portrait filtering on – jimi hendrix filter:portrait
4. with new black and white filtering on – jimi hendrix filter:bw

The feature works pretty well as long as you enclose the name of the person in quotes and use the image search tab. For example, check out the queries

• "don box" filter:face,
•  "robert scoble" filter:face,
•  "olusegun obasanjo" filter:face, 
• "dave winer" filter:face,
  
• "danah boyd" filter:face,
  
• "kanye west" filter:face,
  
• "justin timberlake" filter:face,
• "vint cerf" filter:face
• "richard stallman" filter:face

Very Impressive. The addition of side panel showing "Related People" is also a very nice touch. This is one of the coolest additions to a search engine I've seen this year. I actually lost track of time while browsing faces when I first tried out the feature. Kudos to the Live Search folks.

PS: It seems I've been totally spoiled by Google's Universal Search because I originally performed my searches on the "Web" tab and thought it was a bug when I didn't get any results. I've already forgotten that there was a time when you had to go to each topic specific tab and search for what you want instead of typing your results in a single box and having the search engine figure out if it was a web search, image search, video search or whatever. I can't wait until every search engine has an equivalent feature.