I've recently flown on Continental with a legal sized roll-aboard suitcase, and a pretty good sized, over stuffed backpack. No questions asked. I almost always fly with a backpack counting as my personal item. I try to look as "unencumbered" as I can when boarding, and never put more than one of my carry on items into the overhead bin.
I think enforcement of the rule mostly depends on the size of the plane, and whether the flight is completely booked or not. Although I might fudge on the "size" of the allowed personal item, I try not to intrude on other's space, keeping the back pack under the seat in front of me.