<p>i have a wordpress blog and want to give people the same user experience for adding comments that is in stackoverflow.  There are a number of comments ajax plugins out there but i can't find a working one that allows you to inline on the main page, go in and add comments without first drilling down into a seperate single post page.</p>

<p>Can anyone help here with either a wordpress plugin or php code to do this.</p>

I think
AJAXed Wordpress
does what you're looking for, among other things:
AJAXed Wordpress
AJAXed Wordpress (AWP) harnesses the power of both AJAX and Wordpress to improve
the user experience, the administration capabilities and the design potential of
any Wordpress based blog. It works on all WordPress versions from 2.1 - 2.6.
Some of AWP’s features include loading posts inline,
inline comments
, threaded
comments, AJAX comment submission, AJAX Navigation, live comment preview and much
more. AWP is endlessly customizable and extensible. Even though AWP provides many
features, you are never forced to use features that you don’t want. All aspects of
the plugin are easily customized through a single Administration panel.
Demo is available here
http://wordpress.mu/
and you can see the inline comments in action. Looks like what you were asking for.