It seems that IE is buggy about disabling form elements dynamically if the change is fired from an 'onChange' event on another element (like a checkbox) rather than 'onClick', although FF is OK. If I set an element's 'disabled' property in a script this way, my version of IE6 doesn't update the appearance of the disabled form element straight away. Only when you try to click it does it suddenly become grayed out. The reverse also happens: a greyed out element is slow to return to its active appearance.
If there really is such a bug I guess it could be a problem. If the user tries to navigate the form using a keyboard and no mouse, presumably this will trigger onchange but not onclick. What is the most professional, cross-browser method of dynamically disabling form elements?