On a new OS X 10.6 Snow Leopard the Huwei E220 USB modem (from Swisscom in my case) didn’t work anymore. The USB dongle mounts and the files on the mounted drive were visible but the Unlimited Connection Manager (UCM) installer contained in the mounted drive crashed always. It’s obviously not compatible with 10.6.
To fix this you need to download the latest version from http://www.swisscom.ch/res/hilfe/downloads/mobile/unlimited/index.htm. It still says on that page that only 10.4 and 10.5 are supported but it works just fine with 10.6, too.