Index reported as corrupt after upgrading to 2005

After upgrading a server from SQL Server 2000 to 2005,
an index was suddenly being reported as corrupt in the SQL Server log
(probably every time an Insert was done). DBCC CHECKDB / CHECKTABLE
reported no errors, with or without the new WITH DATA_PURITY option.

Anyone else experienced something similar?

How can an index be reported as corrupt when DBCC doesn't report it?

Dropping and re-creating the index solved the problem.
I've restored a backup of the database (made before dropping/re-creating
the index) to try repeating the problem, but no success so far.The only data purity issue I have had in SQL 2005 so far has been with an XML index on a set of XML documents that may or may not be well formed. The developers are rethinking their strategy at the moment.