Arutz-7 reports:

Archaeologists excavating north of Jerusalem have found a piece of a sarcofagus – a stone coffin – belonging to a son of a High Priest.  The visible inscription reads, “the son of the High Priest” – but the words before it are broken off.  It thus cannot be ascertained which High Priest is referred to, nor the name or age of the deceased…
The precise location of the find is not being released, for security reasons. 
The sarcophagus cover fragment – 60 centimeters (2 feet) long by 48 centimeters (19 inches) wide – is made of hard limestone, is meticulously fashioned, and bears a carved inscription in Hebrew letters that are both similar to today’s script and typical of the Second Temple period.
A number of High Priests served in the Temple in its final decades – it was destroyed in 70 C.E. – and there is no way of knowing which one is noted in the fragment.  Among the known High Priests of the end of the Second Temple period were Caiaphas, Theophilus (Yedidiya) ben Chanan, Shimon ben Baitus, Chanan ben Chanan and others…
Other discoveries at the site include public and residential buildings, agricultural installations, pools and cisterns.

Tombs from the 1st centuries B.C. and A.D. are very common in Jerusalem.  There was a large upper class that built lavish stone tombs, approximately 1,000 of which have been found.

The full story (and a tiny photo) is here.

UPDATE: The Israel Antiquities Authority press release includes a link to a zip file with three high-res photos, including one of the excavation site and two of the inscription. 

HT: Joe Lauer

UPDATE (10/8): Haaretz has the story with some new details, and the Jerusalem Post has a 2-minute video about the excavation and discovery.