I am looking to find out when a SQL Server stored procedure was last
updated. A bonus if I can also find out who did it! I see that the
sysobjects table has a create date & a ref date column, but I could
not find a last update date.
We are running the 'current' versions. I can dig up specific service
pack numbers if required. We normally use Query Analyzer / ALTER
procedure to make changes

Unforunately there is very little support for this in SQL server. The
sysobjects table contains a column called schema_ver, this increases
in value after every change however the date is not recorded. Unless
you have already got a routine that runs daily to log these changes
you can't really see when it was last updated. Do you have any daily
backups, may be you can restore these systematically to a dev server
until you find the change you are looking for, this would at least
give you a date, unfortunately probably not a user.
Best of luck.

Guess the answer is to avoid ALTER PROC and use DROP / CREATE instead.
My problem with this approach is that I have to re-do all the
permissions each time

Here you go:

USE [databasename]
SELECT routine_name, last_altered FROM information_schema.routines
WHERE routine_type = 'PROCEDURE'

It does not capture who made the update but it definitely has the date and time.