The BBC Doctor Who Blog has a wonderful three part interview with Production Designer Michael Pickwoad about the new TARDIS:
Question: What’s your favourite element of the new TARDIS interior?
Michael Pickwoad: This has to be the contra-rotating time rotor. It came to me when realising that all TARDIS have had a large circular feature above the console, which never actually did anything. By making it revolve it would suggest that it was computing the time co-ordinates and setting the course through time. Looking at the revolving tray in a microwave suggested the idea that if each ring supported the next on wheels fixed to the centre, then by turning one ring, the next would revolve in the reverse direction and give more of a sense of computing and conjuring up the idea of a circular slide rule.
Each ring of the rotor is divided into eighteen parts, complimenting the eighteen ribs of the TARDIS structure, and being finished in silver and furnished with Gallifreyan symbols, adds to the sense of precision.