HBO have contracted linguist David J. Peterson of the Language Creation Society to work out a functioning grammar and vocabulary for the Dothraki language for the first season of Game of Thrones. A 1,800-word vocabulary is being developed for the scenes set on the eastern continent in the first season. The LCS has also said that if HBO want them to work on other fictional languages for the series (such as Qartheen or Valyrian) they will be happy to do that as well.
This is interesting news. Whilst the language is not as developed as Tolkien's two elven languages or Klingon, it is comparable to Robert Jordan's Old Tongue from the Wheel of Time books. Given that in the Song of Ice and Fire books themselves there are only a bare handful of Dothraki words given, it is impressive that the LCS have managed to develop a functional (if sparse) language out of them, and is another sign that HBO are taking this project very seriously.