Chris, Opera and IE 7 do not perform actually resizing of the text. These browsers zoom everything on the page so the actual size of the text stays the same. So for these browers detecting text size changes is not very useful. However it is useful to know the size of the text at the point of load (to detect users who need larger fonts for whatever reason - health or hi-res screens) which it does report correctly though.
The article does allude to this behaviour for Opera and IE7 but perhaps we should have made it clearer.
Thanks for posting the error in reporting the -1px decrepancy. Obviously this shouldn’t happen and I’ll take a look (didn’t notice this in testing). The event shouldn’t really fire for Opera and IE 7 but because of the bug, it thinks there is a font size change and fires.