From BBC News.com:
The metre-square, three-toed track was discovered in the Badlands of Montana, US, in the Hell Creek Formation. The size of the footprint (76cm) suggests that it is a T. rex.
There is one previous claim for a T. rex footprint in the scientific literature. The track was discovered in New Mexico in 1983 and published 11 years later.
Update from Darren Tanke (15/10/07): "I was shown a disarticulated tyrannosaur skeleton near Edmonton by the finder; then I showed Tom Owen,who then showed it to Tess, who found the skin impression. There was no ichnite, just a skeleton."