I have a Motorola cell phone, and when it is plugged into my linux laptop via a USB cable it appears as a /dev/ttyACM0 device. (Unless I already have an ACM device plugged in, then it appears as /dev/ttyACM1).  BUT, it's much nicer to be able to point my phone programs (such as kmobiletools and moto4lin) at a logical device, such as /dev/mobile where the number won't change than to point it at the kernel device name.

You can make an automatic symlink from /dev/mobile to whatever actual /dev/ttyACM* device the device appears with using a simple udev rule. Edit a new file in your /etc/udev/rules.d directory named mobile.rules and add the following as a single line:

BUS=="usb", KERNEL=="ttyACM*", SYSFS{product}=="Motorola Phone*", SYMLINK+="mobile"

This tells the udev subsystem that whenever a new device that has a ttyACM kernel device name and a product ID of "Motorola Phone" appears on the USB bus to create a link to it named "mobile" (as this is udev, the full link will be /dev/mobile).

Now, if you only have one mobile phone, this may not be terribly useful. But how about if you have an MP3 Player and a Camera which both appear as /dev/sda1 or /dev/sda2 when you plug them in? Now you can make a logical name for each device, such as /dev/camera-fs and /dev/mp3-fs. You can even write a rule to automatically mount these devices to /mnt/mp3player and /mnt/camera when they are plugged in!

For more details about all of the great things you can do with custom udev rules, see Daniel Drake's excellent tutorial.