Myriad was released in 1992, before the general appearance of extra-light weights, so I'm with Scott on this, it's [EDIT based on Hynes' answer] a custom variation of it.
There is a close-ish match available for purchase that is just for web at WebINK.
It says it's Myriad Pro Light, but if you look at the 24px size one in Chrome (Win 8) it's actually quite a close match:
Apple is using images for the iPad mini texts in their site, this might be on purpose. There is a 'request' in Typophile for a thinner Myriad, you can add your signature to the petition :)
Here's a list of extra light fonts that might be used as replacement.