Yes, I missed the entire topic (mainly because I'm more or less ignoring that board... I'm not that good with themes).
I reported it here because in the Italian board has been reported that with Chrome (12.0.741.122 and XP) the right end of textarea goes far outside the visible area and reverting to the RC5 code fix the problem.
I don't know anything about Chrome, so I don't know if it is relevant or not.
It works fine in Chrome and XP. Was tested on that combo, among others. Yes, the code change you noted in the OP was intentional. Yes, it was because of bloody IE (as most things are).
Like all complaints about this so-called "regression" the problem will be caused by one of two things: either the user did not clear their browser cache after the upgrade, or they are using an older custom theme which has not had the required markup and/or css changes made correctly.
The requirement to clear cache has already been tracked as a known issue in 2.0: http://dev.simplemachines.org/mantis/view.php?id=4750
Editing custom themes after upgrading from release candidates is standard practice anyway.