Tao or Dao? YES!

A friend asks, "Is it spelled "dao" or "tao"?

The answer, of course, is: YES!

In order to translate the phonetics of another language to English, you have to map different letters in the alphabet to the sounds of the language. This is called Romanization:
In Chinese there are anywhere from 5-10 Romanization schemes, depending on how they're counted.

The most popular schemes in the west are the Wade-Giles and Yale:
In both schemes the "dh" sound is Romanized as a "t," thus: tao

The other scheme, Pinyin, (the one I learned growing up in Taiwan and Hong Kong) is actually more popular among Chinese and in pinyin, the "dh" sound is spelled "d", thus: dao.

Thus you get the two different spellings of "Tao" and "Dao," correctly noted in wikipedia.

Of course, there is a great DEBATE about which scheme is "right," but Lao Tzu (or Lao Ji, Lao Tsu, Tse, Tze, Ze, etc!) smiles at all the sound and fury. Isn't it only appropriate that there would be two spellings for a non-dual approach to the world!