Size matters but weight does also!
Late last year we had to fly from Denver International Airport to Europe.
Upon checking our luggage, the clerk asked to weight my wife's carry-on.
It was smaller than the Lowepro Roller 1 Michael mentioned above, but it was one pound too heavy (14lbs)!
He told me that British Airways had a strict 13lbs limit for carry-on.
He also mentioned it was for security reason as a heavy bag could kill a passenger if the overhead bin latch failed. (What about the 255lbs beverage/food carts with sharp metallic angles?)
So we had to check her bag. I was carrying my gear in a medium daypack and I managed to hide it somewhat under my jacket... I had about 25lbs of photo gear with me!
In London Airport it was worse. A security officer was weighting each carry-on by hand just before we boarded for the next flight. Again I was lucky...
For travellers, I've found a link that my be useful:http://www.frequentflyer.oag.com/traveller...arryonguide.asp
It looks like BA and some other companies have set a higher weight limit.