I can understand the confusion with this question since msdn refers to IN many different ways. A number of msdn links have already been posted to defend different answers and here is another one.

http://msdn.microsoft.com/en-us/library/ms177682.aspx

In the remarks section the first sentence refers to IN as a clause. "...values (many thousands) in an IN clause can..."

Also, the article referenced is specifically for IN (Transact-SQL) not predicates or logical operators.

Dave