Incorporating Sibelius into PT sounded like a good idea when I first heard about it. What they have done, apparently, is to include a stripped version of Sibelius in PT8, with no (easy) way of sending the edited score material back and forth between Pro Tools and Sibelius.
It can, however, be sent from PT to Sibelius. But tracks can also be sent to Sibelius from other apps - either via XML, as MIDI files or via a pdf->XML transition.