Get the node ID from the "node/add" menu item and hook_form_alter() I want to set a unique 7-digit code to a custom content type and display it in a disabled textfield when a node is created. I created a CCK textfield and set it to disabled in hook_form_alter, great. 

I think that a good way to make this unique ID is just to use the node id and fill it out to the required length (eg. 0000012). It doesn't look like $nid is available at the node/add form though. How can I set this ID field to default to the $nid? Can I access the hook_form_alter() as the content is created (on form submission)?
You're right, the nid is not available at this point, it's assigned only after the the node is saved.

The easiest solution is most probably to use hook_node_insert and resave the node after having added the value to the field you have created.