The site looks great! I think "Guest Posts" sounds nicer as a page title, and then have a little reminder in the sidebar to "WRITE FOR US! CLICK HERE!" to take you to that page.
If I were you, I'd put another separate contact form on the "Guest Post" page but I'd call this form something else (such as "The Idea Box" or "Input Area"). It's off-putting to have to go to another page to fill out a form. Wufoo.com is great for setting up this type of (free) form which you can then embed onto a page using the html code (which they provide).
At the moment, I'd also say you have too many written instructions; you could give an example guest post to get things started, and perhaps you could show a reply from yourself too. This will start a real conversation faster. Also, the Wufoo form allows you to have instructions which are hidden until the user clicks on a field (e.g. "Subject") which removes the risk of clutter from your page.