Cardinality rule for bitmap indexes
There was never a rule that bitmap indexes were only useful on columns that had relatively few distinct values.  That was a myth that derived from the fact that bitmap indexes aren't appropriate for columns that are unique or mostly unique and that a lot of the columns that you would want to put bitmap indexes on happen to have relatively few distinct values.

Richard Foote (who probably knows more about indexes in Oracle than any other person on the planet) has a nice article on bitmap indexes with many distinct values that walks through why this is perfectly reasonable and appropriate in much more detail.  A followup article comparing bitmap and b-tree indexes on columns with many distinct values is also well worth reading.