Yes, please do! I can't recall a more interesting Easter Egg than this, even better than the developer lake in Southern Kalimdor.

Pretty sure it's tied to the same mechanic that determines when Onyxia Deep Breaths.

After seeing this mentioned in yesterday's 'The Daily Quest' column here, I went to investigate around the web and take a look myself.

According to the blogs that posted this and wohead, this seems to be on a spawn timer of about 6 hours, probably starting to count after they fey-drunk darters have sung.

I flew over there at midnight last night, and was incredibly lucky, as the darters were there and started singing withing 30 seconds of my arrival! :)
I went there again today because I wanted to show some friends, and saw the first darter spawn at 13.45h. It then took exactly 3,5 hours for all the darters to appear and to start singing. A new darter pops up every ten minutes or so, until there are seven. And then.. you wait ;)
I've also seen reports of them starting to sing 10-15 minutes after they have all spawned, so I am guessing it could be anywhere in a 70-210 minute period.

(Ugh.. excuse the typos.. *wowhead* *the* *within* ... I can spell, I swear. Also.. longingly awaiting the new comment system that hopefully has an edit option..!)