SQL execution log

you can see when was the last alter in information_schema.routines

in order to see who did it you will need some tool that can parse the transaction log files

Log Explorer from Lumigent will do the trick for you to see who altered it.

I ran this script

select * from information_schema.routines where routine_name ='usp_Get_pal_On_Trail'

and it returned a record with creation and modification date from march 08 where as i altered this procedure last evening. Strange!