How to create a GET-form For example a filter form on admin areas can best be GET forms, that way they are bookmarkable and can filters be maintained across pager-pages. 

How can one best use form_api for such forms? Will the _validate hooks be submitted? Can the _submit hook be omitted?

For example views-exposed filters work this way; But the code in views handling this is convoluted and complex, too complex to be used as good example for when developing custom interfaces with simple filter forms.
Use the #method parameter of the form like this:

&lt;?php
  $form['#method'] = 'get';
?&gt;