[jodrellblank]

Not only is there not enough space to fit powerful enough magnets

Make it an electromagnet powered by the USB host/power supply, and use the iPhone accelerometer (or more than one) to detect being yanked by the attached cord and disable the magnet to disconnect easily.

(or do what everyone else is and have an inductive charging matt with no connector).

[mynameisvlad]

In order to save space, you suggest... Electromagnets? That would require even more space.