Yesterday the United States District Court in the Western District of Wisconsin declared that the National Day of Prayer is an unconstitutional government endorsement of religion. Though I'm generally a big fan of keeping church and state separate, I'll reserve comment on the particulars of this case until I read the ruling (found here, as a pdf). Until then, here are some links to articles I found helpful:
Howard Friedman's Religion Clause blog: National Day of Prayer Declared Unconstitutional
Baptist Joint Committee for Religious Freedom blog: Judge Rules National Day of Prayer Law Unconstitutional
The Christian Science Monitor: Federal Judge: National Day of Prayer is unconstitutional
Also, I wrote about the National Day of Prayer a few years ago, here: Why I don't like the National Day of Prayer
The Lutheran Zephyr 