It's expected that you let the person who's been waiting the longest move there. That's why you'll hear the new cashier say "I can help the next person in line." I consider hopping over from the back of a line to be very rude.
I've also seen clueless managers at some stores (cough cough TARGET) take the people in the back of the lines and redirect them to the newly opening lanes. Pisses me off to no end when I'm the guy that's been waiting the longest and grandma in front of me pulls out her checkbook....
