Brian Swan has a new post answering a question he's gotten about the stored procedures that the SQL Server database includes and whether or not they help prevent SQL injections in your applications.
When I’ve asked people about their strategies for preventing SQL injection, one response is sometimes "I use stored procedures." But, stored procedures do not, by themselves, necessarily protect against SQL injection. The usefulness of a stored procedure as a protective measure has everything to do with how the stored procedure is written. Write a stored procedure one way, and you can prevent SQL Injection. Write it another way, and you are still vulnerable.
The short answer is "not always" but he gets into a more detailed answer with a sample login script and the SQL to create the stored procedure the "wrong way" (using the value dynamically in the SQL of the procedure) and the "right way" (assigning them directly like bound variables).