preset the position of new frame under mac os x

[Leo]

hi there

i want to position new frames in the left half of the screen. coming 
from nt-emacs i just tweaked default-frame-alist with (left . 100). 
unfortunalty do new frames still appear in the left side of the screen. :-(

so, default-frame-alist doesn't seems to work properly. strangly an 
entry for left in initial-frame-alist is recognised!

does anybody know a workaround or so?>

thanks, leo