I'd also like to add that it works under Ubuntu 9.10

For me I simply plugged in the N900 using a micro-USB cable, then selected PC suite mode on the N900 when it came up. Under Ubuntu I then clicked on the network connections. It showed the Nokia N900 (PC Suite Mode) and under it I was able to set up a new GSM connection. It took all of like 15 seconds and then I was able to connect and use it. Very slick.

divinerites: It should be a very simple matter of adding a ppp connect and use the device of /dev/ttyACM0. You might need to look up and see what device gets added when you plug in the N900 and select PC mode. Either a dmesg or tail the /var/log/messages file should show something.