Outlook doesn't even do threading,
Not exactly on-topic or indeed of interest to anyone here, but Outlook 2010 actually does come with a new conversation view (it's been rolled out at my workplace). Not that I particularly like the way they've implemented it, tho.
This is, coincidentally, at the same time that Google have said they'll finally provide an option to disable their own conversation view in GMail - something they've been unwilling to do for several years, despite user requests.
Peter.