From the http://www.transactsql.com site, great t-sql reference over the books online:

DBCC EXTENTINFO ('@dbname', '@tablename', @indid)

Example

--Display information about the extents in the authors table

--Clean up the display

SET NOCOUNT ON

--List information about the first index in the authors table

DBCC EXTENTINFO ('pubs', 'authors',1)

--List information about all the indexs in the authors table

DBCC EXTENTINFO ('pubs', 'authors')

--List information about all the indexs in the pubs database

DBCC EXTENTINFO ('pubs')

GO

This didnt do it for me, I believe the extent number must be embedded in the header pages that bitmap off the free and used extents, but even so, im still unsure how to extract this info. The command above is close to useless really in terms of extent numbering (logical or physical).