it's not thinner its actually .5mm thicker
1st generation: 110×61.8×8 mm (4.3×2.4×0.31 in)
2nd generation: 110×61.8×8.5 mm (4.3×2.4×0.33 in)
source: http://en.wikipedia.org/wiki/IPod_Touch
Exactly, although, parts of it are thinnner.
It is much thinner at the back edges, not at at the middle.
That and the fact that it's contoured are the main reasons people think it's thinner.