Looking at the code I suppose ui.resizable is meant to support preservation of aspectRatio either by specifying the aspectRatio = true option or by pressing the SHIFT-key.
However most of the handles are really buggy when the SHIFT-key is pressed especially the N/NW/NE-handles... Also I would expect from the E-handle that - while the SHIFT key is pressed - the resizable resizes to top AND bottom and not only to the bottom...
I know that it is quite difficult to properly implement such an aspect ratio preservation since I tried it myself and failed. A lot of margins have to be recalculated over and over again. But I remember that Interface had a properly working implementation, so maybe just have a look at the way they calculated the margin's there?
I think this is a pretty essential feature since it adds so much use to the resizables (e.g. without it an image-cropper where a given aspect ratio is essential will not be possible). I would highly appreciate it if this feature would make it into the 1.5 beta!
I've also described this problem in #2145 but since the basic funcionality has been implemented I think it's no enhancement anymore but a bug.