Now that the iPhone 5 details are public, I thought I'd redo the math for the weight-per-volume using those new numbers.
The iPad 3 is 25.7 cubic inches (in³) and a weight of 23 oz. For 0.89 oz/in³.
The iPad 2 is 23.6 in³ and a weight of 21.3 oz. For 0.90 oz/in³.
The new iPhone 5 is 3.38 in³ and a weight of 3.95 oz. That's a slightly higher 1.16 oz/in³.
Using the predicted iPad mini/air size in inches of 7.78 x 5.31 x 0.28 is 11.57 in³....
Using an iPad density gives a weight of 10.3 oz (292 grams) on the low end and an iPhone 5 density give 13.4 oz (380 grams) on the high end.
Even the higher number is less than the weight of the new 7" Kindle Fire HD (13.9 oz).
I'm still guessing closer to the iPad density and a bit over 10 oz. But even on the high end, its still less than the competition.