Actually, it's quite simple. It's exactly the same as running the xconfig rootfs from TI-Planet but with a debian rootfs.

Create the rootfs with debootstrap (Thank you Lionel for the script) :
# Make sure we have the appropriate packages.
apt-get install debootstrap multistrap binfmt-support
# This package ought to register the appropriate definitions for binfmt-support
apt-get install qemu-user-static
# Load binfmt support
modprobe binfmt_misc
# Start a debootstrap that will fail when trying to install the packages
mkdir $DEST_DIR
cd $DEST_DIR
debootstrap --verbose --arch=armel sid . http://ftp.fr.debian.org/debian/
# Fix it. ln assumes the programs are on the same partition, use cp instead if they aren't.
ln /usr/bin/qemu-arm-static ./usr/bin/qemu-arm-static
# Start debootstrap again, and while at it, we can add some packages into the mix :)
debootstrap --verbose --arch=armel --include=locales,build-essential sid . http://ftp.fr.debian.org/debian/I have replaced sid by testing which will become stable in a few months
Plus, I have added xpdf and the xorg metapkg

And then, I used qemu to finish the installation, create the root password and install xcas : http://wiki.debian.org/QemuUserEmulation (appendix)

Once you have your rootfs, you can put it on your pen drive
If you succeed to run X, let me know !

Okay !
Debian is installed and it seems to works ... but I encountered some type of a problem ...
When I run dpkg --configure -a, I get an error :
dpkg : error : unable to access dpkg status area : No such file or directoryIs this behaviour expected ?

@Vogtinator
Sorry, I don't understand your edited answer ...
- If I use OTG, the calc mode will be device if I use a 4-pin mini-usb cable or host if I use a 5-pin (with a bridge between pin 5 and pin 4) mini-usb cable. And the mode can be switched without rebooting the calc.
- If I don't use OTG, the usb mode will follow that rule (and will not follow the "cable rule") :

USB support is mostly working. The only thing missing is a USB PHY driver. The USB controller driver is currently unable to switch seamlessly between USB host and USB device mode because of this. Unfortunately, nothing much is known about the USB PHY.
That's probably the same reason why sometimes USB devices aren't recognised in USB host mode - the USB PHY hasn't changed the USB hardware to host mode.
To work around this, we get the Nspire OS to set the right modes in the USB PHY before booting Linux.
For USB Host mode: insert a USB OTG cable's A end or a USB device into the calculator while still inside the Nspire OS before running the bootloader.
For USB Device mode: connect the Nspire to a computer while in Nspire OS before running the bootloader.

Or there is something that I don't understand ?

@Dapianokid
I don't see why It should work with a type A instead of a type B when TI uses type B connectors ...
I'm not even sure that we can use a type A male connector with a type B female connector ...
I think that the parameter is "cx_usb_otg" because it's written on hackspire

edit :
Okayyyy
It's written usb type A in my quotation !!!
There is definitively something that I don't understand  

edit n°2 :
So, if I understand well, it is not possible to use an usb key without an OTG usb cable ?!?
If I put a 4 pin mini-usb cable, nothing happens to the usb PHY who keeps is previous state which is device mode ... It makes me very unhappy  
But I still don't see why I have to use a type A usb instead of a type B usb ...

Hello.
I would like to confirm that I understand well the usb support matter ...

1) I plug the usb while in the Nspire OS then boot linux => the nspire is the host and will keep the host status until I reboot it.
2) I plug the usb after booting linux => the nspire is the device and the usb (computer ?) is the host. I can't use an usb key in this mode.
3) I use the cx_usb_otg kernel parameter and the host/device status of the nspire depend of the cable I'm using (if it is an OTG cable the calc will be a host and if it a normal cable the calc will be a device).

Is it right ?

I'll try to remove the clutch screws after my homework gets done.  
Thanks.

After those screws should the top come right off?  Also do the clutch screws need to be put back in?

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Oh yeah !!!
I suceed to remove the clutch screws with a 1.2 mm flat screwdriver. It's was so easy with the right tool

Maybe there are hidden screws under pads ... I will see that later ...

edit :
Okay that's it ! You have two more clutch screws under the top pads. Once you have removed them, push the battery area to remove the motherboard part

_Nicco_  , did you succeed to remove the Clutch screw (near the battery) ?
I bet that theses are clutch but I'm not sure, they are so tiny ...

@fortytwo
I know the torx I hoped that i've had this size but unfortunately, it's not the case. So I think that I will use a flat screw driver for theses ...
Nevertheless, I was talking about the helix shaped crews

@Dapianokid
Yep ! I used this trick on my gamecube
I don't know why but I was reluctant to use it on my cx ...
But, thanks to you, I will use it and my cx will be taken apart until the evening !