This is a bug in the RadListBox that we are aware of. It is logged into our bug-tracking system, however up to now its priority was quite low, therefore it is still not fixed. I will increase it's priority, so that we will come into fixing it as soon as possible.
Just to mention, it is currently possible to use the client side api to enable/disable items under both FireFox and IE. Example code:
This will disable an item, detaching all its handlers and active styles. As you may speculate, using enable instead of disable will lead to the item being enabled.
All the best,
the Telerik team