From dictionary.com's prayer entry, definition 1b: The act of making a reverent petition to God, a god, or another object of worship.

W also referenced other religions: Every day, millions of us turn to the Almighty in reverence and humility. Every day, our churches and synagogues and mosques and temples are filled with men and women who pray to our Maker.

I don't know what efforts the President made to invite non Judeo-Christian religious leaders, but that would be the Presidents prerogative anyway. The Constitution states Congress can pass no law giving preference to any one religion. The President is not specifically excluded from showing preference for any (or no) religion. If he was, he would be unable to attend the religious services of his choice, violating his freedom to worship as he pleased. The president (or any members of Congress, or government workers) is no less a citizen of the United States as any other citizen.

I really wish people would quit thinking everyone is out to do $BAD_THING to everyone else. Relax, enjoy life. If you really have a problem with the President's National Prayer Day gathering, write to the President, Congress, and the Supreme Court.