When I shop, I ask for a bag of "Sidamo coffee" because that is how it is branded, notwithstanding the ongoing debate over the political history of the names. I recently read about the history behind the two terms and the complexity of the issue. The political side of this question is discussed here: http://www.sidamaconcern.com/news2007/0810071.htm
My understanding has always been that "Sidamo" is the name of the region inhabited by the Sidama people. Further, I used to think that "Sidamo coffee" was named after the region where it is grown much like Harar coffee, Yirgacheffe coffee. Apparently, this is not the case, at least according to this article: http://www.americanchronicle.com/articles/view/44986
Proponents of the campaign to reject the use of "Sidamo" argue that the term is not only a misnomer but also the result of a deliberate and derogatory acts of "the invading Abyssinian soldiers of King Minelik as part of the campaign to humiliate, undermine and subjugate the newly conquered territories in the South of the country." In recognition of this, the government and ECX officially changed late last year the coffee name from Sidamo to Sidama.
But, changing a widely known brand (even if it is replacing "o" by "a") may not be that easy. The good thing: the general public, coffee businesses, and consumers know the name "Sidamo" as a coffee name - a reference to one of the finest coffees originating from Ethiopia - not in its derogatory sense. In any case, as far as I know, there is no marketing campaign underway - or planned - in Ethiopia to "educate" the world about the name change and to drive the brand to a soft landing at "Sidama coffee".
I hope this helps...