usb gsm modem not working if plugged in after bootup

[Tomoki Taniguchi]

I have a USB GSM modem.
the device is recognized and a ttyUSB0 device is created properly
if the device is connected prior to booting my machine. (I am using gutsy)
but when I plug the device in after the machine has booted up,
the ttyUSB0 is not created.

can anyone help me?  i hate having to reboot each time
i decide i want to get on the internet.

TIA,
Tomoki