SQL 2005 Stored Procedures Last Modify Date

USE Database --Change to the database you want to use
SELECT name, create_date, modify_date
FROM sys.objects --User defined objects system view
WHERE type = 'P' --Only return stored procedures
AND modify_date > '2009-09-01' --If you aren't looking for a specific data range, take this line out
ORDER BY modify_date DESC

Read sys.objects documentation

Also read documentation on Querying the SQL Server System Catalog, it contains some information on backward compatibility and future releases of SQL Server.