National Geographic does this pretty regularly cuz I see pictures on yahoo all the time. I think it would be fun to enter one time...
http://news.yahoo.com/nphotos/The2010-National-Geographic-PhotoContest/ss/events/lf/112310natgeophoto#photoViewer=/ydownload/20101123/photos_net_web_yn/1290540476
This one is my favorite:
http://news.yahoo.com/nphotos/The2010-National-Geographic-PhotoContest/ss/events/lf/112310natgeophoto#photoViewer=/ydownload/20101123/photos_net_web_yn/1290540476
This one is my favorite: