One would set the mouseout to disabled = '' instead.
<input type="button" name="test" id="test" value="roll over me" onmouseover="this.disabled=true;" onmouseout="this.disabled='';">
The disabled property only looks to see if it's there at all. One can set disabled='anything' and it will be disabled. Furthermore, add padding to the SPAN tag and will allow the events to work properly. Without padding, it will not trap the events because the input button is disabled. In the code below, a green background is added to display the SPAN area.
<span style="padding: 8px; background: green;" onmouseout="this.firstChild.disabled='';"><input type="button" name="test" id="test" value="roll over me" onmouseover="this.disabled=true;"></span>
Hope this helps!
Source: "Javascript: enable/disable button with mouseover/mouseout"