I’ve got the following code: http://jsfiddle.net/69MqZ/1/
As you can see grey text lines up to grey meta data: 
I want it to be like this despite length of name to the left: 
Obviously it has to use position: relative or some technique that will make border-bottom move accordingly. This is comments output generated by WordPress.
I’m facing the same issue here (all containers have float: left;):

You will need to wrap a parent element around both the comment author name AND the meta data – .comment-author & .comment-meta – and then float THAT parent element. e.g.:
Then
That will allow the comment text to come up all the way to the top of the area like you intend.