I have just released The reddit button Plugin for WordPress.
You can use this plugin to insert a reddit button for your post.
This plugin automatically adds a button to the QuickTags toolbar in the post screen. Clicking the button will prompt you to insert the style of the button you wish to add.
If you are not using the Quicktags i.e. using the WYSIWYG or you want to do it manually, you can add
<!--reddit_3--> where you want to display the button. For correct XHTML, make sure that the code is added on a new line.