Isn't that the whole point for the practice of lean startups? Where you validate that there is a market by asking people to give their email address before you go and start spending time and money on it.
Still feels a bit "icky" to me, but then I like the act of building stuff itself.
I wouldn't be surprised if it's all a massive hoax to see how many people you can get, with a little clever marketing, to sign up for a site that doesn't even exist.
Though now that it's been so successful, it'd kindof be a waste not to monetize the hoax.
1) Landing page with email capture + vague promise of something cool
2) ???
3) Profit!
I wouldn't be surprised if they don't have a product yet and are still figuring things out...
EDIT: Just realised I am a hypocrite... http://littlecosm.com (I assure you that there is a product behind this, though!)