Not too long ago blizzard announced that they're working on a system that will allow you to change factions for a price (and not a gold one at that). Much like many other character customizations it will now be possible to literally become your enemy changing yourself into a race of the opposite faction that can carry your class.
That said it will still not be possible to change your own race within your own faction and even switching to alliance from horde and back will not allow you to actually end up with a different race (the system remembers your previous race).
So there we have it: You can turn your Tauren Shaman into say a squid shaman (ok ok draenei for all you pro-squid folk) and back but there will be absolutely no option to turn your tauren into an orc shaman.
I have to admit this confuses me a little. I can understand the reservations blizzard may have in letting you change your own race to a certain extend because they assume that people will then always switch to the race with the 'best' racials.
But on the other hand I find that concern to be somewhat of a moot point. I am sure there's a certain amount of people that would switch for the purpose of racials but given the overall state of racials the number of people will definitely be less than 10% of the population.
Add to that the fact that people value different racials and the net effect will be very limited in terms of racial shifts (maybe a few more undead) meaning that it won't upset the race balance but will result in a nice little cash flow for blizzard.
This and the general thought that by allowing a faction switch you will still more or less be able to pick the best racials (except they'll be in a different faction and from a limited selection of races based on your class) and the whole thing becomes more of a faux argument than anything else.
Honestly the primary drawback for faction switching I see is that a lot of hordies I know would switch to alliance purely to escape the dysmal economy we have due to a 3:1 alliance:horde size difference. Which in turn would result in the already bloated alliance faction to become even bigger. Now I don't mind a bit of a tenacity buff, but being outnumbered 4:1 every place you go might be a bit over the top.
Maybe only alliance will be allowed to switch factions since they're traditionally the larger faction on most servers.
I don't know how big the percentage of people is that would dish out for faction changes, I am sure a fair share of gold sellers, scammers and grievers will find the option enticing (assuming it's cheaper than a realm transfer) but even then it'd be interesting to know how many would choose to become their own worst enemy by switching sides.