From the Faction Change Service FAQ:
Updated* 7/1- Due to the many questions that have come up with the announcement of this new service, we wanted to take the time to answer the ones we can at this point in time.
Q. Will we be able to switch between the races on our own faction?
A.. No. Players will only be able to switch to a race of the opposite faction.
Q. Will I be able to choose the race on the opposite faction that I want to change to?
A.. Yes, but you will only be able to switch to a race that has your class type available to it. So if you play a human paladin, you’d only be able to change to a blood elf paladin.
Q.How much will it cost?
A.. We do not have further information on this at this point in time.
Q.Will I be able to switch back to my original faction but a different race?
A.. No. You will only be able to switch back to your originally chosen race.
Q.. How will the switch between reputation, gear, mounts, etc be handled?
A.. We’ll have more details for you at a later point in time, though we plan to keep these as close to a reflection of the other faction as much as possible.
Q.How often can you change your faction?
A..We do not have any information to share on this at this point in time, however we will have restrictions on the frequency by which players can change their faction.
Q. How will this affect the balance of Horde and Alliance on the realms?
A.. We are taking great care in how we implement this new service in order to maintain balance between the factions on the realms but do not have any further details to share.
No Troll Paladins? Gnome Shaman? I'm now totally against this service :(.