Slate.com might as well be a Kerry campaign office. An internal poll of all their 51 employees and contributors, similar to one conducted in 2000 had the following results:

45 for Kerry; 4 for Bush; 1 for Michael Badnarik (Libertarian); and 1 for David Cobb (Green).

That being said, the reason for this disclosure is that they want to prove that they can be fair, regardless. I don't read the site often enough to know.