Ah, now this is just type geekery gone bonkers.
Kern is the new iPhone game from FORMation, who are an "alliance of independent creators" based in Texas.
The game involves moving a letter along a base line so that it's in exactly the right spot to fit into the word that's dropping down onto it from above. And that's pretty much it. But it's been designed by Jason Franzen with a typographer's keen attention to detail, and it only costs 59p, so even if you only play it a few times (most likely in the pub when you're trying to outgeek some other type nut), it's hardly breaking the bank.
Ridiculous yet essential.
Thanks to Catherine Dixon for the heads up.