It's a very old issue, a frustrating one in fact: the edit tab on nodes disappears sometimes due to input format permissions. I wrote a module that fixes this for D6 which works for the default body field , CCK textareas and I'll be adding support for other textareas on other pages too (think blocks & views). It's available in my sandbox for testing at http://cvs.drupal.org/viewvc.py/drupal/contributions/sandbox/swentel/edi.... There are a few todos left, marked accordingly in the code, so if you want to help out, go nuts! I'm not sure yet if I'll release this as an official project on d.o, I'll wait for some reactions from the community first, so leave a comment, contact me, ping me on irc .. we'll see after that.
Anyhow, say hi to the edit button again if you test it :)