In reply to jugpuller:
Lowe Alpine TT Carry On 40 is a travel style rucksac. It fits 'most' carry on requrements. I make a point of not filling it to the top - then I can fold down the top 5 cm to make it fit the size gauges if it is a small one.
It fits more than I need - did a 4 week tour of India with one.
I got the missus one so we could have his and hers luggage in different colours - very chi-chi!
The main thing is the sac needs to be as light as possible or it eats into the max carry-on weight.
I reckon there is a market for a suitcase / rucksac with rigid sides made from a light plastic that is exactly the size of a smaller carry-on allowance. That way you could really squish stuff in.