There are White Label plugins for WordPress. Although I'd love to see It being implemented officially.
What would also be awesome would be the ability to change the login page (you can already do this through httaccess I know, but It would be better If It was implemented).
Also, the ability to change the names of the 'wp-content', 'wp-admin' etc folders.
I love wordpress, but sometimes It doesn't look professional to reveal that the website was created with wordpress (even If It's only mentioned on the source code);