You most likely know that since HTML Elements in Internet Explorer are wrappers around
COM objects, you can't use HTMLElement.prototype, unlike Firefox (or another
Mozilla based browser). Jason Davis has
a presented a solution to this in his post, Element
Prototyping in Internet Explorer 5.5 and above, where he used a behavior to provide
this missing functionality.
It would appear that there is a memory
leak and possible
performance issues (which I was not aware of) with the use of behaviors,
so way up your options carefully before employing this approach.