If more than half of the pictures on your Flickr account are screen shots, Flickr will suppress your images in global search results. The theory is that people come to Flickr to see photos, not group shots of your World of Warcraft guild or the sculpture you just created in Second Life.
This is strictly a business decision. Either Flickr knows their users as well as they think they do, and this is the right call, or they’re cutting off a potential market for no good reason. Which is it? Beats me. The one thing that’s certain is that Flickr is a much better application for sharing screen shots than anything else I’ve seen, for the same reason it’s great for sharing photos.
If images could be categorized as screen shots, Flickr could just add an “include screen shots” flag to searches, so people could specify the kinds of things they care about. I wonder whether it would be difficult to write an image processing algorithm that automatically differentiates between photos and screen shots? I’d bet that it could be done.
Updated: See the comments for the final word on the subject from Flickr head honcho Stewart Butterfield.