In mailing lists I have found a trick, but is not that clear how to apply it. I worked on it, and at the end, I correctly understood how to make the suggested change. To make things easier, here is a patch:

 *** erlang.el.orig      Mon Aug 11 18:17:02 2008
 --- erlang.el   Mon Aug 11 18:56:36 2008
 ***************
 *** 5726,5730 ****
   compile command `c' when using the option `outdir'.
   
 ! There exists two workarounds for this bug:
   
     1) Place the directory in the Erlang load path.
 --- 5726,5730 ----
   compile command `c' when using the option `outdir'.
   
 ! There exists two workarounds dirfor this bug:
   
     1) Place the directory in the Erlang load path.
 ***************
 *** 5740,5744 ****
     (or (inferior-erlang-running-p)
         (error "Error starting inferior Erlang shell"))
 !   (let ((dir (file-name-directory (buffer-file-name)))
   ;;; (file (file-name-nondirectory (buffer-file-name)))
         (noext (substring (buffer-file-name) 0 -4))
 --- 5740,5748 ----
     (or (inferior-erlang-running-p)
         (error "Error starting inferior Erlang shell"))
 !   (let* (
 !       (buffer-dir (file-name-directory (buffer-file-name)))
 !       (ebin (concat buffer-dir "../ebin/"))
 !       (dir (if (file-readable-p ebin) ebin buffer-dir))
 !       ;(dir (file-name-directory (buffer-file-name)))
   ;;; (file (file-name-nondirectory (buffer-file-name)))
         (noext (substring (buffer-file-name) 0 -4))
 

To apply it,  on a Mac, select the code before and copy in the clipboard. Then do:

cd /usr/local/lib/erlang/lib/tools-2.6.1/emacs/
cp erlang.el erlang.el.backup
patch

Now paste, then type ^d.
Now you should see

patching file erlang.el
Hunk #2 succeeded at 5740 with fuzz 1.

Done! Now when you compile, IF exists ../ebin, the beam will be placed there.

If you are using a not-unix platform or you are too lazy to apply the patch, simply download the attachment and replace the erlang.el file.