The Cornell Lab of Ornithology is holding its annual GBBC on the weekend of February 18-21, 2011. Here's their webpage for information on how to take part and submit you lists, and also look at previous years' results: http://www.birdsource.org/gbbc/
Last year 97,331 checklists were submitted, 603 species of birds were seen and an incredible 11,233,609 individual birds were counted.
You don't have to limit yourself to just your backyard, but can go out to parks, nature reserves, State and National Parks, etc., so make the best of the 4 day President's Day weekend holiday, everyone.
And don't forget your camera, as there is a GBBC gallery to upload your photos to - they like to see the people taking part as well as the birds you see.