hook_form_alter execution order
Also worth mentioning, there is a new drupal 7 API called hook_module_implements_alter() which lets you alter the execution order for a given hook WIHOUT altering the module weights table. 

Sample code from the API docs showing how easy this is to do:

&lt;?php
function hook_module_implements_alter(&amp;$implementations, $hook) {
  if ($hook == 'rdf_mapping') {
    // Move my_module_rdf_mapping() to the end of the list. module_implements()
    // iterates through $implementations with a foreach loop which PHP iterates
    // in the order that the items were added, so to move an item to the end of
    // the array, we remove it and then add it.
    $group = $implementations['my_module'];
    unset($implementations['my_module']);
    $implementations['my_module'] = $group;
  }
}
?&gt;