I've got a Sony P11Z netbook with a built-in USB 3G Modem and i'm using debian, maybe my problem is the same as yours.
# lsusb -s 001:007
Bus 001 Device 007: ID 0af0:7601 Option Globetrotter MO40x 3G Modem (GTM 382)
When booting a 3.10 kernel the mobile broadband option usually is missing (but not always). When i boot a 3.2 kernel it shows up again (most often).
I checked /var/log/syslog while restarting network-manager and found these 3 lines:
Dec 4 08:31:18 plok NetworkManager: <info> rfkill0: found WiFi radio killswitch (at /sys/devices/LNXSYSTM:00/device:00/PNP0A08:00/device:17/SNY5001:00/rfkill/rfkill0) (platform driver Sony Notebook Control Driver)
Dec 4 08:31:18 plok NetworkManager: <info> rfkill2: found WWAN radio killswitch (at /sys/devices/LNXSYSTM:00/device:00/PNP0A08:00/device:17/SNY5001:00/rfkill/rfkill2) (platform driver Sony Notebook Control Driver)
Dec 4 08:31:18 plok NetworkManager: <info> rfkill3: found WiFi radio killswitch (at /sys/devices/pci0000:00/0000:00:1c.1/0000:02:00.0/ieee80211/phy0/rfkill3) (driver ath9k)
Now i found out that, in my case, issuing the following command would bring up the device, regardless of the currently running kernel.
It takes a few seconds to initialize (follow the process with tail -f /var/log/syslog), then the "mobile broadband" option would show up in gnome-network-manager.
echo 1 > /sys/devices/LNXSYSTM:00/device:00/PNP0A08:00/device:17/SNY5001:00/rfkill/rfkill2/state