If you are using IE5,5.5, or 6 - you'll probably be seeing this in a tabbed version, In IE6, you'll see the first letter underlined, and you'll be able to use alt-letter to access that tab - alt-letter should also work in IE5 and 5.5 - but without the first-letter support, you won't be able to see it.
You should be getting the fallback content. Whilst this would be possible in NN4 - it would compromise the fallback situation so much, that I don't feel it's viable to offer it - you'd hurt more people than the current version.
This should probably work, I've no idea if the Alt-letter does, I've not checked if Accesskey is supported - I doubt it - If not when I eventually test on Mozilla, I'll probably modify it so it does - you should be seeing the first letter underline. Of course you could be seeing the fallback content - if you are then I've got something wrong with my Mozilla predictions - if you're seeing garbage then I've gone very wrong! - I'll correct it when I have a mozilla to check.
Opera can't do this easily, it would be possible, again like Navigator 4, except it would actually be saver, but it would take a lot more care and would be a more "custom" solution on each page.
These should all be seeing the fallback content - unless they support the DOM methods getElementsByTagName and getElementById and the innerHTML extension.