Damien: I'd be interested in hearing more detail.
A bunch of clarifications:
- displaying results in a POST (on form submission) is just a bad
idea. Don't do that, you should use GET.
- if you really want to display results in a POST (remember: don't do
that), use the form submit function for what it is designed to do:
take action. In your case the action is to rebuild the form and
display the result there: store the results in $form_state['storage'],
and display that in your form callback.
Damien