Here's an excerpt from the Press Release by the Israel Antiquities Authority:
A unique discovery was revealed in excavations that were conducted north of Jerusalem: a fragment of a sarcophagus cover was found that is engraved with square Hebrew script, characteristic of the Second Temple period. The fragment (length 0.60 m, width 0.48 m) is made of hard limestone, is meticulously fashioned and bears a carved inscription that reads: “…Ben HaCohen HaGadol…”.
Numerous high priests served in the temple during the latter part of the Second Temple period and there is no way of knowing which of the priests the inscription refers to. However, it should probably be identified with one of the priests that officiated there between the years 30 and 70 CE. Among the high priests we know of from the end of the Second Temple period were Caiaphas the priest, Theophilus (Yedidiya) Ben Hanan, Simon Ben Boethus, Hanan Ben Hanan and others.
UPDATE (10/6/08, 9:30 pm): I've corrected my misspelling of "sarcophagus" in this post. Also, Jim Davila has posted the news story, and Ed Cook has offered some thoughts on the orthography of the inscription.
[HT: Jack Sasson, Agade Mailing List]