Youre wondering why you cant use your n900 in ubuntu 11.04 as modem?
Here is a quick and dirty fix to get GSM only connection:
rmmod cdc_acm modprobe cdc_acm
or you can use wvdial; here my config:
[Dialer Defaults] Init1 = ATZ Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0 Init3 = AT+CGDCONT=1,"IP","business.gprsinternet" Modem Type = USB Modem Baud = 460800 New PPPD = yes Modem = /dev/ttyACM0 ISDN = 0 Phone = *99***1# Password = t-mobile Username = tm