I usually drink double espressos and they always seem to taste much better out of a ceramic cup compared to take-away paper cups. I ordered 2 back to back from the same shop within half an hour yesterday, one in a ceramic cup and one to go (I have a coffee drinking problem...) and the difference was still there - so same barista, machine, beans etc. Why is this? For sushi you are told not to eat it with a metal fork since it will supposedly taint the taste, so could this be similar?
Both cup material and temperature are in play here, I think.
A big part of the coffee taste (and experience as a whole) is the aroma. I personally find that paper cups (not surprisingly) smell and taste like paper. Ceramic (or glass, porcelain, etc.) should be practically neutral. Any smell you get from the cup could affect the perception of taste of the contained beverage. If the cup has a plastic lid, that could contribute both to its own smell, or holding-in / reducing the coffee aroma that you smell, also. (Aside: there are some interesting links to the physiology of smell and taste interplay at the Wikipedia article on taste.)
The very experience of drinking from a paper cup versus a ceramic cup is different; heft, shape, handle, other aesthetics, etc., though this probably reduces to personal preference. For example, I find sometimes that my lip sticks to the outside of the paper cup, which is unpleasant (to me, anyway :). A plastic lid could be a factor here as well.
Temperature may also be a factor. For example, temperature can affect the perception of bitterness. A properly pre-heated ceramic cup/demitasse will help the brewed coffee be at a consistent temperature (without shocking it after pull) and keep it warmer for longer (so it doesn't cool down too quickly). Such temperature changes could result in a difference in taste; keeping brewed coffee at a consistent temperature is recommended -- that and some other general properties are listed here. Temperature stability of the brew will probably be different between paper and ceramic.
Keep in mind that paper cups are manufactured. I have been in industrial plants my whole career. There are odors, vapors and minute particles in the air that get packaged with the product. They are not harmful per say but are released into the hot beverage on contact and could likely impart a flavor.