This is an interesting idea! And one which will seem like a godsend to digital designers. When I was at The Times creating layouts for the iPad and Android, it was such a pain having to create every layout twice, for both portrait and landscape orientations.
A ‘liquid layout’ feature would make life so much easier - things would get done much quicker and you would no longer have to put up with ugly borders around a layout as a result of it not fitting a certain screen size. The perfectionist in me loves this.