The N9 use KDE, which comes with another command shell.
It is usually fully possible to install KDE on Debian, then change make the "Harmattan" window manager.
90% of the code on the N9 is available on the N900. Had it been up to me, I would leave the KDE to the advance users, and develop a real extension of Hildon, not just another variant. E.g. allow the user to touch two points at a time to pull points away. There is also a pile of possible improvement in "transations", e.g. fad-out or "flip around" a cube. We have 4 screen now, a dice has 6 sides: Place phone on 1 and Contacts on 6, Conversations on 3 and email on 4 - then scroll around a virtual dice. Do not mimick others, allow people to improve!