okay, nevermind, i figured it out myself.
need to hack "ob-ditaa.el":
look for the line (cmd (concat "java" .... and insert your cygwin path
(e.g. c:/cygwin) just before shell-quote-argument,
org-babel-process-file-name in-file and org-babel-process-file-name
out-file, so that it will look like this:

(cmd (concat "java " java " -jar c:/cygwin"
      (shell-quote-argument
       (expand-file-name org-ditaa-jar-path))
      " " cmdline
      " c:/cygwin" (org-babel-process-file-name in-file)
      " c:/cygwin" (org-babel-process-file-name out-file))))

hopefully this will help someone.

On Tue, Jan 3, 2012 at 11:05 AM, Waldemar Reusch <
waldemar.reusch@googlemail.com> wrote:

> http://www.mail-archive.com/emacs-orgmode@gnu.org/msg21593.html
>
> im trying to set up ditaa on my cygwin/windows7 machine and encountered
> the very same problem as described in the above mentioned mail, i.e. the
> error "jar not accessible".
> i tried to fix it by hacking org-exp-blocks, but it does not seem to make
> any difference. when i check the *Messages* buffer, the path to ditaa.jar
> is unchanged.
> If anyone has this one figured out, i would appreciate your help.
>
>