I agree, there doesn't seem to be any mention in the documentation about it. And it's not just zero increments - you can't have a seed or increment which has a higher value than the maximum for the data type, or lower than the minimum (hence you can't have negative increments with tinyint identity columns). You can't have fractional values for either seed or increment, etc., etc.

All semi-obvious, but as yet I can't find any official documentation.

Duncan