Going by Engadget, a bit thinner than the iPhone 4 supposedly.

30% difference

N9 - 12.1 mm [1]

iPhone4 - 9.3 mm [2]

[1] http://europe.nokia.com/find-products/devices/nokia-n9/speci...

[2] http://www.apple.com/iphone/specs.html

N9: 7.6-12.1 mm

iP4: 9.3-9.3 mm

In all honesty, that sounds more comfortable to my hands. One of the reasons I have never switched to iPhone 4 from the 3gs was because I never really liked how it felt in my hands without a case (I used them all day for mobile testing)

if you measure iP4 depth at Power button, it will be even less (~3 mm?). Makes about same sense as 7.6mm for N9.

Apple specifies Macbook Air thickness like this [1], so I think it should be fair game for the N9.

[1] http://www.apple.com/macbookair/specs.html

I thought that the OP meant actual device thickness, not the marketing tricks employed by Apple or Nokia.