Tested it on IE 6 and got: "Tests passed"
So I guess it works . . . :-)
I don't have access to IE5.5, would love to have someone test it.
IETester hooking is not perfect yet, but the engine is the correct versioned engine used on each tab. You can find more information here :
http://www.my-debugbar.com/wiki/IETester/HowDoesItWork
Regards.