Album art has never been perfectly centered on the iPhone or iPod touch Lock screens. It's been placed immediately on top of the slide-to-unlock bar. Because of the 3:2 aspect ratio of previous iPhone and iPod touch devices, however, and because the top of the album art and the reflection beneath it peak through the translucent time and slide-to-unlock bars, the visual weighting was close enough to appear centered and thus, balanced. Not so with the iPhone 5 and its 16:9 aspect ratio. Now the bottom alignment of album art lookes decidedly bottom heavy. Worse, there's a big black gap between the top of the album art and the bottom of the time bar.
The advantage of bottom aligning album artwork on the 16:9 display is that none of it is obscured by the name of the song, and even when you double-click the Home button to bring up the music controls, very little is obscured. The disadvantage is, in design parlance, "Oh god, my eyes, my eyes..."