Regarding the statement that you could see changes in the dates when you used CREATE, but not when you use ALTER, with the queries provided in this tread...
What version of SQL Server are you using? I don't have a place to confirm anymore, but I seem to remember that in SQL 2000 (and maybe 2005), the DateModified value didn't get changed in the system tables when a stored procedure was ALTERed.
Of course, my memory could be way off, too. But it may be worth a quick test.