This is actually available if you have Gmail, and download the Gmail Plugin... but there's a problem with this, since the plugin acts as only an inbox, it doesn't show whenever you've sent a message, it only shows the incoming ones, and any subsequent sent messages under that subject heading. If you send a new message, the sent item goes to your messages folder, and doesn't show in your e-mail folder... it's a huge fail in my book.
I think threaded email would be a huge help for business email puposes. For all BlackBerry's can do, it seems rather foolish that this is not built into the BB experience from the factory.
Would Anyone Else Like To See This Become Reality?....
I don't want to archive old emails on my BlackBerry. What a storage nightmare. I use Gmail and love it. The Gmail app allows you to go back and look at old email and that works perfectly for me.