In a thread on the forums posters are listing evidence for this.
When going to the "jedi-knight" url on the holonet you get a page stating "You are not authorized to access this page." This text indicates that it is a valid URL for upcoming content, as otherwise it'd just give a blank page (and previously revealed content had such a text as well before it was released).
But more telling is the following picture people uncovered in the website's directory structure:
That subtitle reads "savior of the galaxy".
Also the following two forum images were uncovered:
The first one of those clearly seems to be a jedi, and the second might be an Imperial Agent or such as many are speculating. But it's unlikely that they'll reveal two classes at once. It looks quite likely that they'll reveal another class this Friday (or soon at least).
Of course, as with all rumors, this isn't anywhere near official until BioWare say that it is official. Though I do recall Sean saying that he wanted to leave such 'hints' around for forum members to uncover, so it's looking likely that we'll get the Jedi Knight class soon.
[UPDATE] It seems that Darth Hater has something more to report on this. Apparently some user found the flash animation for the Jedi Knight, making it as good as certain that this will be the next class reveal. Even more telling is that it seems that both this Flash animation and the header title image (I reported on above) have been removed from the site, making it clear that BioWare didn't really want those to leak just yet (too late). I've replaced the image above with one someone managed to save and put on flickr, but believe me it was on swtor at first.