Also in BOL under the CREATE TABLE entry is the following (partial) definition of IDENTITY (emphasis mine):

IDENTITY

Indicates that the new column is an identity column. When a new row is added to the table, the Database Engine provides a unique, incremental value for the column. ... The IDENTITY property can be assigned to tinyint, smallint, int, bigint, decimal(p,0), or numeric(p,0) columns.

increment

Is the incremental value added to the identity value of the previous row loaded.

Since the engine creates a unique, incremental value, it must be a whole number value which fits in the bounds of the data type, and is other than zero.