Usability: Disappearing edit tabs due to input formats

Splitting this issue off from: http://drupal.org/node/243728
and http://drupal.org/node/110242

As stated on #243728, if a user has access to edit x content type, but doesn't have access to the filter format of that post, they lose the edit tab with no explanation. This has the effect of very confused support requests from site users (I've personally had many), and I've probably answered 4-5 support requests in the past month or so from Drupal users with the same issue.

The issue manifests itself in a few ways:
1. users have permission to edit any [content type], but can only edit some
2. admin users/editors edit a node using a different input format, and the user loses access to their node.

In both cases, there's no access denied or warning to inform the user that they no longer have access to edit, they just lose the edit tab.

Additionally, there's nothing to warn editors when changing an input format that the owner of the node doesn't have access to it.

The filter format priority is a good one, and needs to be kept.

So instead I think we've got a couple of options:
1. leave the edit tab but disable text areas which users don't have permission to edit - this would mean they could still change titles and stuff, and would have an explanation of what exactly the issue was.
2. add a small javascript warning to the input format selection which informs editors that they're choosing a restricted input format - either not available to the user or not available to auth users or whatever.

I'm marking this as critical because it's the cause of a lot of support requests and confusion. Making input formats into a first class permission will help, but I think we can go a step further.

Here's the results of a quick google search for disappearing edit tabs:
http://drupal.org/node/126504
http://drupal.org/node/15283 <<- note the nid.