1-A primary key must be defined on the table.
2-The index cannot be on indexed views.
A spatial index is for columns that are of the geometry or geography types and requires that a primary key be defined, of which there are less than 15 key columns. There cannot be a spatial index created on indexed views, but the table does not need a rowversion column or a clustered index.
To know more http://msdn.microsoft.com/en-us/library/bb964740(SQL.100).aspx