Sorry if I am mistaken. I never tried this out but as per the MSDN article
http://msdn.microsoft.com/en-us/library/ms177456.aspx it says this
The data rows of the disabled clustered index cannot be accessed except to drop or rebuild the clustered index. This means the following:
These operations will fail: SELECT, UPDATE, DELETE, INSERT, CREATE INDEX, CREATE STATISTICS, UPDATE STATISTICS (on the index), and ALTER TABLE statements that modify table columns or constraints.
So in that case, wont the statement fail when we try to alter the table to drop the constraint??