I've looked at a number of pages so far:
My understanding based on the above is that the Sierra Wireless drivers are already included with non-ancient Linux kernels, and the device should be automatically detected, with some
/dev/ttyUSB* device files being created as a result. I don't see these files, however.
dmesg shows a couple lines like
[ 555.877426] usb 2-2: config 1 has an invalid interface number: 7 but max is 5
which appears to suggest that the device is in its software installation mode, rather than modem mode. I'm not sure how to get the device into modem mode; my understanding is the driver is supposed to be doing this anyway.
I've tried plugging in under Ubuntu 12.04 and 10.04.04, with basically the same results (i.e., device not detected). I was able to
modprobe sierra and
modprobe sierra_net (on 12.04 at least; 10.04.04 didn't seem to have the
sierra_net module), so the drivers should be included.
The device did work on a couple Windows 7 machines I tried it on, though a funny thing is that it didn't automatically install its drivers like it was supposed to; I had to go download them from AT&T's website and install them myself.