I haven't seen a weight limit enforced domestically for carry ons (and in the last year we've flown, we've made multiple trips to Seattle, San Jose, Dallas, Orlando, Tampa, Atlanta, Los Angeles, Chicago) We have had the 50lb rule applied to our checked luggage though, but usually were able to rearrange between the two of us.
Internationally, is a different story. Our flights to/from UK and to from UK/Paris they were very strict about the carryon weight limit, but didn't seem to mind me taking my two water bottles out of my carryon and putting them in my purse (??) British Air, Virgin and BMI.
This is the kind of thing you might want to ask over on flyertalk - the folks there are often business travelers and travel far and wide on different airlines.
--heather