The billboard reads "God is an imaginary friend. Choose reality, it will be better for all of us." Below the main message, it attributes the sentiment to the Colorado Coalition of Reason.
MORE: http://www.9news.com/news/local/article/244639/222/New-atheist-billboard-raising-eyebrows
See also...
Atheists rejoin billboard battle in Colorado
http://coloradoindependent.com/110749/atheists-rejoin-billboard-battle-in-colorado
Atheist billboard sparks debate in Denver
http://www.washingtonpost.com/local/atheist-billboard-sparks-debate-in-denver/2012/01/24/gIQARuHjNQ_video.html
New Billboard In Springs Calling God An Imaginary Friend
http://www.krdo.com/news/30279411/detail.html
Upcoming Atheist Billboard Draws Large Response
http://denver.cbslocal.com/2012/01/24/upcoming-atheist-billboard-draws-large-response/
Atheist Group Pays For Billboards That Read: 'God Is An Imaginary Friend'
http://www.huffingtonpost.com/2012/01/23/atheist-group-pays-for-bi_n_1224500.html
Atheist billboard causes stir in CO
http://www.wivb.com/dpp/news/local/atheist-billboard-causes-stir-in-co
Boulder atheists buy Springs, Denver billboards
http://www.gazette.com/articles/springs-132204-colorado-boulder.html
Billboard: God Is An Imaginary Friend
http://www.kktv.com/home/headlines/Billboard_God_Is_An_Imaginary_Friend_137941593.html
Atheist billboard raising hackles among conservatives in Colorado Springs
http://www.therepublic.com/view/story/af0c600529a14909b679c9347e488c66/CO--Atheist-Billboard/
http://unitedcor.org/national/news/cocore-colorado-billboards-god-imaginary-friend