I would also love this feature, there is a work around for you to install "any" app for any other OEM market place, however when I tried installing the HTC app that includes the clock tile, it failed because the Nokia 920's resolution isn't supported
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Also because live tiles are limited to being updated only every 20 minutes maximum, only hardware manufacturers such as HTC, Samsung or Nokia have the greater access to the OS to make this possible.
So in brief, if this is a feature you'd truly love, just let Nokia know as I did. The more people to request a feature, the sooner it will come!
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)