I'm not quite getting the picture of your page layout, da_pinky, but I would expect that putting it above the fold and in the content area would be best. Making it look like a button and using a contrasting color would help.
If they would be replying to some other poster, placing the button right under the post would likely work well.
You might also try tweaking the wording... "Post" vs. "Post Now" vs. "Post Instantly" etc. Or maybe a different term than "post" depending on your audience.