Funny that. I've just had exactly the same problem with an Elsa microlink USB modem.
There seems to be something wrong with the USB in 7.3. I have two USB devices: the modem and an Epson Perfection scanner. The scanner works with no problems. On the surface all seems well with the modem: it is recognised by Linux and listed in the /proc/bus/usb/devices file, the acm module and the other USB modules are loaded. However, any attempt to read or write to the /dev/ttyACM0 modem device gives an "Invalid argument" error. I checked the /proc/bus/usb/devices file again. The acm driver is not associated with the modem. OTOH, the scanner driver is associated with the scanner. So, the USB subsystem is successfully loading the acm driver but is not associating it with the modem device. No idea why, and no idea how to fix it :( The modem worked with Suse 7.0 and the 2.2 kernels and 2.4 kernels up to 2.4.5, with compiled-in USB support. The USB documentation says that if no driver is associated then it means the modem is not ACM-compliant. I think USB is broken. Peter