With the most recent updates to raspbian (Wheezy) I found that USB modeswitch was no longer switching my Huwaei mobile dongle from usb-storage mode to modem mode. Rather problematic when your endpoint is on a boat and you are trying to connect to it remotely.
After much digging around I think it might be to do with how the kernel usb-storage handles the device. Aferr much mucking around, and this post on stackoverflow , I sussed out that by adding the -I flag I could get the switch to happen as expected. The -I flag sets InquireDevice, so you can make the change more permanent by editing /etc/usb_modeswitch.d/12d1:151a to the following:
# Huawei E392u-12 TargetVendor= 0x12d1 TargetProduct= 0x151b InquireDevice= 0 MessageContent="55534243123456780000000000000011062000000100000000000000000000"
This now works reliably over reboots for bringing the device into modem mode.