Okay, here are my testing experiences on Mac OS X 10.2.
Mozilla 1.5rc2 - works as expected; scrolls page down to Quick Reply field and activates it so you can type in it.
Safari 1.0 - actually activates the Quick Reply field, but fails to scroll the page down to it, giving it the appearance of not working.
Internet Explorer 5.2 - Takes you to the old, regular reply page even when you click on the Quick Reply button (and yes, I tried several times to make sure I wasn't accidentally clicking in the right place).
Hope this helps,
--Scott M.