The trick to making an open office plan work is to use the lack of walls to increase the square footage per employee. On top of that, it really helps to have "break-out" spaces or quiet rooms, where you can go to get some work done away from the bustle.

The problem is that it's easy to fall into the trap of cutting costs as part of transitioning to an open plan. You really can't have it both ways -- either you increase productivity and pay more in square-footage, or you cram everyone into a much smaller space at the cost of productivity and happiness.