Captain Cloudhammer has served the Equestrian Royal Guard for as long as anypony can remember. He has seen more threats to Equestria come and go than most should, from Nightmare Moon's return to Discord's revival. But when a threat unlike any other arises to threaten the very nature of Equestria itself, and forces the Bearers of the Elements of Harmony to travel to a strange and unknown land, Cloudhammer volunteers without hesitation to escort them. But the journey will test him in ways he never thought possible, and teach him lessons in friendship not soon forgotten.

Artwork commissioned from CrunchNugget on DeviantArt: http://crunchnugget.deviantart.com/

Robert Parker is a 24 year old ex-web developer living in New York city. In the wake of the conversion movement, he has seemingly lost everything. Haunted by the same nightmare every night, he soon realizes that he must make a choice.

To those unfamiliar with the Conversion Bureau: it is a scenario created by an author named blaze in which Ponies either appear in the human world or have lived beside humans without incident for millennia. One day, in the face of all the damage humanity has done to its world, the ponies offer a solution: undergo conversion to become a pony and allow the world to be inundated with Equestria's influence: thus healing it and bring forth a golden age.

Many different authors have taken many different approaches to this idea, all of them are worth checking out.

http://www.ponychan.net/chan/fic/res/53616.html (The current thread)
http://www.equestriadaily.com/2011/04/conversion-bureau-compilation.html (The compilation post on EQDaily. *NOT UPDATED*)