This error indicates that the page has an incorrect value on it for the reserved space. In SQL 2000 it was possible for the row and page counts for a table or index to be incorrect, even negative. CheckDB did not pick this up. On SQL 2005, the counts should be correctly kept and CheckDB gives a warning when it finds this scenario
This is not a serious problem and it's trivial to fix. As the message says, run DBCC UPDATEUSAGE on the database in question and the warnings will disappear. This is common on databases upgraded from SQL 2000, for the reasons mentioned above, and should not occur on database created in SQL 2005/2008.