On second inspection, C-M-; is not bound by default.

How would you feel about binding the following function to that  
keybinding?

(defun comment-dwim-line (&optional arg)
   ""
   (interactive "*P")
   (comment-normalize-vars)
   (if (not (region-active-p))
       (comment-or-uncomment-region (line-beginning-position) (line- 
end-position))
     (comment-dwim arg)))

Obviously a better description than "" is needed, but the behavior of  
this has changed a bit since it can now afford to (being a different  
function and keybinding). Rather than relying on the position of  
(point), it has a clear established behavior of commenting out the  
current line unless the region is active.

On Dec 9, 2008, at 2:48 PM, Stefan Monnier wrote:

>> Generally speaking, what makes BOL a "common starting point" to  
>> perform
>> comment-indent more-so than any other part of the line?
>
> I'm not sure it's much more so, but it's at least as common as
> current-indentation, end of line, etc...
>
>> Additionally, is not BOL semantically the clearest place to run  
>> comment-dwim
>> and expect it to comment out a given line rather than having it run
>> comment-indent?
>
> Could be.  I'm not saying your idea isn't good.  I'm just saying  
> that it
> is not compatible with the current behavior and that the current
> behavior makes sense as well.  You may find some other way to combine
> comment-indent, comment-kill, and comment-region onto a single key.
> E.g. M-; M-; is currently unused.
>
>
>        Stefan