Hello,
please excuse my bad english ...
Since the wifi hotspot on the N9 only works in ad-hoc mode, I'm trying to connect through USB tethering and it still does'nt work.

I'm on Archlinux and the network-manager does recognize the usb modem, it gives me an IP adress and i can ping or ssh the phone but it looks like there is no gateway for accessing the Internet ...

Maybe in there is some commands I have to execute (devel-su mode or ... ?) on the phone in order to set it as gateway ?

I love this phone but I really need to be able to use the 3G on my laptop, as it's my only internet connection and I'm a freelance developer ...

Thank you in advance.

yes, if you search the forum, you will find the instructions.

I did search the forum and I found instructions talking about Windows and Nokia Suite but nothing for Linux ...
I try again.

On gnome this is easy.
1. Right click on the network manager applet in the tray.
2. Go to Edit connections...
3. Click on the mobile broadband tab.
4. Click add and follow the instructions

I think my N9 misses some software because when I plug it in usb, I can only choose between 'mass storage', 'connect and sync' or 'sdk'.
None of those allow the network manager on my computer to recognize the N9 as a mobile broadband device. It just create a local wired connection between the phone and the computer.

I still can't find the right thing to do ...

Ah, I forgot to mention that. You need to choose connect and sync. The local wired (usb) connection does come up for me as well. You need to setup the mobile broadband connection in addition to this. Follow the above instructions for setting up. Make sure enable mobile broadband is checked.
http://www.youtube.com/watch?v=bqffjU0_BUg

Correct. I can confirm it works, I was discussing with Harmattan mates about this and found this post. The wired1 connection that creates on PC (ethernet) it works on Android but not on N900 or N9 so by creating a mobile broadband access point you can use tthose phones modems over usb, Actually I tested a MVNO sim this way on my N900 and Debian. You have to know which are your operator's parameters to create the access point while on wired you don't know have to which apn you are using, the connection manager detects it.
BR
SteP