This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Sam, In terms of performance, both should be the same. I have tried it in db2 and Oracle, so I know that the optimizer does the same thing in those databases.
Having said that, you should use a prepared statement so you gain the efficiency when repeating the same number of in clause parameters. For example, for one dept, your string would be: SELECT * FROM dept WHERE dept_name in (?) For three depts, your string would be: SELECT * FROM dept WHERE dept_name in (?,?,?) and so forth.