In the US you never write a thesis, you write a dissertation (document) based on a thesis (idea.)

Interesting. Wikipedia thinks the two are synonymous:

"In academia, a thesis or dissertation is a document that presents the author's research and findings and is submitted in support of candidature for a degree or professional qualification."

[from http://en.wikipedia.org/wiki/Thesis]

Npk is correct, but it's a pedantic distinction. Most people, apparently including the Wikipedia editors, just use the two words interchangably.

Edit: yikes. Somebody went on a downmodding spree on this thread.

I asked all my academic friends, and I've come to the conclusion that it is a pedantic distinction. But it's useful to have two different words for the two separate concepts :)