Themed post editor

I want to extend my theme to have an embedded post editor – that is, I want a user to be able to edit a post within the themed main site without going to the administration panel.

I’m familiar with theming, and I’m pretty sure I can’t add this functionality through straight editing of the templates. I’m also familiar with PHP (but not with the WP functions), but if someone could give me some pointers on where the functions/content in the code is for the editor and any tips on how to go about doing this, it would be most appreciated!

Thanks!
Kent