get-a-post and WP 2.0.4

The plugin works with 2.0.4. The error you’re getting is described here:
http://codex.wordpress.org/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F

The problem is an extra line (or white space) in a part of the plugin’s file where it shouldn’t be.

If you originally just copied the source of the plugin to another file, I’d recommend instead downloading the zip, extracting the plugin’s script (get-a-post.php) from it, and uploading that to your blog.