I'm still not sure what was 'hijacking' the symlink, but I've got a fix by adding these lines to /etc/udev/rules.d/udev.rules:
# Motorola USB cell modem
KERNEL=="ttyACM0", NAME="%k", SYMLINK="modem", GROUP="tty"
Now the symlink /dev/modem will point to /dev/ttyACM0 even after a restart. Tried this method after reading the info in:http://www.linuxformat.co.uk/pdfs/LXF66.tut_udev.pdf
Not sure if this solution is proper, but it does work in this case!