I found a way to make it work - I think!

In the file /lib/udev/rules.d/61-option-modem-modeswitch.rules, installed by the udev package, there is also a rule for my Huawei modem: ATTRS{idVendor}=="12d1", ATTRS{idProduct}=="1446", RUN+="modem-modeswitch -v 0x%s{idVendor} -p 0x%s{idProduct} -t option ...

Fine, here is the new log:
http://di-vers.dyndns.org/usblog2

And now the connection does appear in the NetworkManager Applet

As far as I know, I'm running a pretty ordinary Ubuntu Karmic Koala

The line indeed disappeared:
USB_ModeSwitch log from Sun May 02 20:18:14 CEST 2010

Using global config file: /etc/usb_modeswitch.conf
raw args from udev: 1-2/1-2:1.0
USB dir exists: /sys/bus/usb/devices/1-2
Warning: USB attribute "serial" not found.
----------------
USB values from sysfs ...

Fine - it is here:

$ lsusb -v -d 12d1:1446

Bus 001 Device 005: ID 12d1:1446 Huawei Technologies Co., Ltd.
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 2.00
bDeviceClass 0 (Defined at Interface level)
bDeviceSubClass 0
bDeviceProtocol 0
bMaxPacketSize0 64
idVendor 0x12d1 ...

Fine, you can get the log-file here:
http://di-vers.dyndns.org/usb_modeswitch_1-2:1.0

I have now upgraded to version 1.1.2, but unfortunately the problem seems to remain the same.

As suggested, enabling logging produces a lengthy log file that I hesitate to post here in total - I will bring what I guess are the some of the most interesting parts:
USB_ModeSwitch log from Sat May 01 ...

I have tried to follow all the instructions I have found here, and I'm very please that usb_modeswitch has made it possible for me to use my Huawei modem on my small eee 901. But I have not been able to make it work automagically - I have to run usb_modeswitch "by hand".

When I plug-in the modem ...