WordPress is a trademark, both in word and image. It is required by international trademark laws to use both appropriately.
WordPress is spelled with a capital W and capital P, all one word. Usages of Wordpress, Word Press, and similar spellings and capitalization is unacceptable and in violation of trademark.
The WordPress logo is a tall, “graceful” W in a circle with a blue or gray background. Other colors and some creative distortion are acceptable, but the font and height must be maintained.
Images courtesy of Matt Mullenweg.