Is the work done walking up an escalator in the same speed and opposite direction of the escalator zero?

yeah,you are right. since you are at the same distance above the ground so you have your potential energy constant so you are not doing any work though if you were on the frame of escalator then you are moving so work is done in form of kinetic energy.

Essentially, what you are missing is that you need to look at your situation from the point of view of the escalator steps. The escalator "sees" you "climbing" it because each step you take places you at a later escalator step. You don't keep landing on the same step.

A force is applied by your foot on each escalator step. Therefore, each escalator step applies the equal and opposite force on your foot. The distance you need to be concerned with is not the distance between your feet and the immovable ground floor. Instead, you need to be concerned with the distance your foot has moved while applying the force to the escalator step.

When your foot first contacts the escalator step, it is at point "A". The force is applied by your foot until it reaches point "B". The force was applied over that "A-B" distance. So work is clearly being done even though your potential energy relative to the immovable ground floor is not increasing. You are working to maintain your potential energy relative to the immovable ground floor while the escalator is working against your efforts.

The overall potential energy change is zero. But as you will have noticed, you get tired doing this. That's because you are doing work on the escalator.

If a person stands on a perfectly efficient friction-free escalator, work will be done on it as they go down, and the esclator would generate electricity. This is the conversion of potential energy into electricity.

If the person walks up as the escalator moves down in order to keep their position constant, we have the chemical energy of their food being changed into electricity.

The original meaning of the word "treadmill" (before the invention of exercise machines) was, by analogy with windmills and watermills, a wheel in which prisoners were forced to climb in order to generate energy for industry. There were various designs, one resembling a hamster wheel. Some other ones are shown here http://en.wikipedia.org/wiki/Treadmill

All the answers so far have started with the incorrect assumption that it's equally difficult to walk up a downward moving escalator and to walk up a stationary stairs. This is simply not true. True you expend some energy as the escalator takes you down a bit and you have to recover that lost potential energy (height) with each step. But if you could hold the bulk of your mass in a constant position and just move your legs and feet - you'd find the effort much less.