I recently had this problem on SQL Server 2005 SP2 and SP3, and in my case the reason was that there were two identical indexes on a table:
-1st was a clustered unique index with column1, column2
-2nd was an index for primary key constraint also with column1, column2
The server raised the error about foreign key reference when I tried to Drop the 1st index. So the only way I found to trick it was to delete foreign key references and then recreate them...