Nokia N900 as 3G Modem
Although N900 is actually linux phone (Maemo is Linux), it not as Linux-friendly as e.g. Samsung Galaxy. However, you can use it for tethering in Linux. You just have to know what to do. Only USB tethering is available out of the box. Bluetooth and WiFi need additional software and little tinkering.
When you connect phone and computer with USB cable choose PC Suite mode from these options. Connection does not work automatically, however (it may but, it is unlikely). All you have to do is open network manager, and create new broadband connection using same parameters you would use if you were using USB 3G modem stick. This varies according to your network provider, so I do not provide any specifics in here. You usually need to know your country and ISP name and possibly some other parameters. You can also use wvdial to activate the connection with these instructions.
Important thing is to notice, that when tethering with N900, you have to think it as a USB modem (just like USB 3G dongle). This differs from e.g Samsung Galaxy S I9000 and Sony Xperia X1, where the phone acts like a router. For these two the a hared network connection is already active, and USB connection is like normal ethernet connection. While plugging the cable in for those, Ubuntu immediately sniffs the presence of active network and gets IP address and other relevan info with DHCP. But with N900, you have to activate a ppp connetion the same way you do with 3G modems.
For this, you need root privileges, and run a script to activate DUN. I’ll explain this later.
You can use e.g. JoikuSpot, which costs less than 10 euros. That turns your phone into a wifi access point.