(I know most folks are flying domestic here, but we do have posters outside the US, so I'll mention it.)
US carriers do not presently enforce weight limits for carryon baggage, but most foreign carriers do; they weigh the bags at checkin. Out of habit, I limit the weight of my carryons according to that standard, which is usually ~12 lbs. In any case, I agree with Carol, if you can't easily lift it overhead, you have no business carrying it on.
For the current limit on the weight and dimensions of checked and carryon bags, always check your airline website, and be sure to check it again right before you leave, in case they happen to change it. On many airlines, the weight limit for domestic flights is different, and lower, than for int'l flights, so be sure you are looking at the right limit.
BTW, you can't try to combine passenger bag weight allowances in order to take fewer bags. One 100 lb. bag for two people is not the same as two 50 lb. bags for two people. Part of the reasoning on the limits is to cut down on Worker's comp. claims from ramp personnel injuring themselves by lifting especially heavy bags. We do like to use one larger bag sometimes, especially when we have one adult travelling with multiple children, or if we have to take a train to our hotel from the airport. When this is the plan, we have taken to putting a duffel inside the rolling pullman. We use space bags to pack the pullman, then when we are ready to check it, take out a few and move them into the duffel, which we check as the second bag. When we claim the bags we just re-consolidate them before leaving the baggage area.
