Comment 12 for bug 796871

No, we have everything:

modem-manager[12298]: <debug> [1308440689.323452] [mm-manager.c:793] device_added(): (tty/ttyUSB0): port's parent device is blacklisted

Looking at this it seems as though this particular Gobi modem doesn't properly gets its firmware loaded, perhaps?

Right now it's blacklisted by /lib/udev/rules.d/77-mm-qdl-device-blacklist.rules:
# Acer Gobi QDL device
ATTRS{idVendor}=="05c6", ATTRS{idProduct}=="9211", ENV{ID_MM_DEVICE_IGNORE}="1"

What you could try to do would be to comment out the line starting with ATTRS above in the file, reboot, and see if the device answers better.