We wanted to see if we could electro-etch $1 travel mugs, and here is what we learned... It is indeed possible! And the results look good too!

Supplies

Salt (any kind)
Water (tap water is fine)
Battery (Safety would suggest a 9v battery or less) or a AC input to 9v or less DC output wall adapter
Alligator Clips
Rubber Gloves
Metal Travel Mugs ($1 from the local dollar store)
Q-Tips
Vinyl or Electrical Tape for the mask

Step 1: Step 1: Take Safety Precautions

Safety First! When working with electricity, it is always best to be sure that you follow some basic safety precautions. This experiment has several potentially problematic aspects, so be sure to exercise caution and observe all safety precautions.

Electricity: Get yourself some rubber gloves and put them on! You will be using water and electricity, so rubber gloves are a very good idea.
Electricity: Insulate yourself from ground by wearing rubber soled shoes.
Hydrogen Gas: This experiment has the capacity to create Hydrogen, which can go boom, so be sure to do the experiment in a well ventilated area away from anything that can initiate a spark or flame.
Chlorine Gas: By using table salt we can liberate the Chlorine and thus create a gas that is very bad for you. Be sure to do this experiment outside and wear proper respiratory protection.
Safety Glasses: Always wear safety glasses and/or goggles.

Step 2: Step 2: Prepare the Mask

Create your mask. We chose to use our vinyl cutter to make a mask, however electrical tape works just fine too.

Apply the mask to your travel mug.

Step 3: Step 3: Prepare the Water Solution and Applicator

Add approximately 1 tablespoon of table salt (NaCl) to 1 cup of water. We used regular Morton table salt and regular tap water and this worked out just fine.
Stir the water until the salt is dissolved.
Dip one end of a q-tip cotton swab into the salt water solution and place the q-tip aside.

Step 4: Step 4: Battery Time

Using an alligator clip, connect the positive side of the 9v battery to the metal of the cup. We used tape to help hold the clip against the wall of the cup.

Using an alligator clip, connect the negative side of the 9v battery to the wet end of the q-tip cotton swab from the previous step.

Step 5: Step 5: Etch!

Touch the wet end of the q-tip to the unmasked area of the travel mug. Slowly go back and forth over the exposed area. The more time you spend on an area, the deeper the etch will be.