Connect N900 to an USB-bus, use "PC Suite Mode"

pyyhttu@dhcppc1:$ lsusb
Bus 002 Device 007: ID 0421:01c8 Nokia Mobile Phones

pyyhttu@dhcppc1:$ ifconfig
usb0      Link encap:Ethernet  HWaddr 32:7b:77:96:59:9f  
          inet6 addr: fe80::307b:77ff:fe96:599f/64 Scope:Link
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:90 (90.0 B)usb0	Link

At this point make sure the connection is offline at the N900's end.

pyyhttu@dhcppc1:$ sudo aptitude install wvial

pyyhttu@dhcppc1:$ sudo wget -c -O /etc/wvdial.conf http://iki.fi/~pyyhttu/debian/N900/wvdial/wvdial.conf

OR

run wvdial config: 

pyyhttu@dhcppc1:$ sudo wvdialconf