I can list at least 20 significant differences.
Don't be shy, now. Of course there are differences, but 20? At least 20? And all "significant" ones?
OP: It's not the carriers' faults. Apple prices the iPhone at a $370 premium over the iPod touch and it's their job to recover it if they're going to offer it for $200 or so. The differences I see are:
GPS
$2
Cellular radio
$25
Better rear camera
I estimate $10 difference based on Touch camera costing $10
Earpieces/speaker
No known $ but let's be safe and say $10
The wall adapter
Less than $5, which is how much the whole box+all accessories cost
8GB more memory
I estimate $15 based on 16GB costing Apple $27
Compass
$1
All numbers from iSuppli
That's $68 in parts. Does anyone think that testing/approval/labor for these 7 things costs Apple anywhere near $300, half the cost of the device?
Give me a break. Obviously Apple has the right to set their own prices and it's working out pretty well for them (hell, I bought one) but you've gotta be pretty darn naive to think the iPhone 4 is priced in reasonable proportion to the iPod touch, especially when you factor in that the iPod touch is 25% thinner than the iPhone.
Of course APple's not alone in this, all smartphones are overpriced which I guess is to be expected for a luxury like a smartphone.