For the 3 main status (OK, warning, invalid) we could use a 'traffic light' arrangement - colour blind people know how to read that.
Using again road sign analogy, I'm not sure a square is the right format for 'invalid' - why not use an exclamation mark ! as it is used in many practical implementation of form checking.
For delayed status an hour-glass is more common (Windows waiting)
I'm not sure the 'internal error' status should be conveyed to the user but if it does a ? could be OK.

I hope you get more feedback and by merging them all you'll get a nice set :-)

Why not let all users enable the icons? Most people can figure out what red/yellow means, but I doubt many would know what a blue/pink colour signal is for.

(From Alex: Hence, the tooltips. :-) )

And for the disabled box, it might make sense to have it grey, or even use opacity tricks.

(From Alex: On disabled widgets, I intend to reuse Mozilla's native settings. Otherwise, I will be inviting disaster by confusion. That said, for any new widgets, I just want to emulate Mozilla's behavior as closely as possible.)

What will these be used for? What is a "delayed" textbox?

(From Alex: Delayed means that the application wanted to let the user finish typing before it attempted validation. Internal-error and warning I don't know if I'm going to use.

But being able to state a control is invalid is pretty important.

I hope to use several of these in Verbosio, but they're general purpose extensions. Some ideas will work, some won't. For example, I'm already revising my approach to textboxes, as I found I couldn't exactly stick one of these images inside the input box. The colors were often too close.)