New info, they changed it to BOP with a high drop rate.
This post is now void.
Once 3.1 hits, Sprite Darter Hatchlings will be farmable from Sprite Darters in Feralas -- I presume that both sides can farm them, but Alliance have their own quest to get one, so farming will be more of a Horde activity (Alliance will join in for the sales opportunities, I'm sure).
On a PVP server, expect the area to be camped by Alliance. Because Alliance are like that.
Depending on how aggressive you are, if you get in and farm this pet when 3.1 first hits, you can sell it for astronomical amounts of gold. To anyone farming this in the first week, I recommend you SELL and do not keep this pet. I also recommend bringing a friend with AOE or quick range (so you can tag more darters at a time), make targeting macros, and make sure you log out in the right spot the night before. If you bring a friend, agree to split the profit.
Over time, the price for these will go down toward Whelpling range, which is why you need to be in the first wave to make the most money. I wouldn't be surprised if the first 5 Sprite Darters listed on Neutral AH and Horde AH net close to 20,000g on high pop servers. No one on Horde has this pet, so the demand is high and the supply, so far, has been nonexistent -- the most perfect recipe for profit you will ever find.
You'll have a lot of people competing with you (or killing you), which is why I suggest you only try this if you're aggressive and willing to fight a little dirty to get the mobs tagged.
To be honest, I don't think farming Darters is a good idea unless you have the ability to stay at it, screw people over, and get lucky in the first few days (I'd take one look at the crowd and hearth dejectedly). You also want to get your Darters up on the AH fastfastfast because once supply starts growing, prices will drop.
For BUYERS -- do not bid on the first wave of Darters. Wait a few months. You will save thousands of gold if you're patient because prices will stay high (though not as high as first-week sales) until the most obsessive Horde collectors are done buying.
For your entertainment, when 3.1 comes out, I'll run out to the Darter area on Zuluhed (low pop PVP) and Moon Guard (high pop Normal RP) and take screenshots so you can see the insanity.