From the original poster's request for a way to make sure podcasts update on start up, I was thinking that creating an applescript that would run on startup would do the trick.
What I left out of the instruction above was to save the applescript as an application. Then drag that application to your start-up items under system prefs.
A Podcast set to update once a day will update its automatic update time to 24 hours from the last update. So, if you issue a manual update at 10AM on a Monday, the next automatic update would be on Tuesday at 10 AM.
Well, if you always want an update at start up, use an applescript to accomplish this. Then you can click on the update button to your hearts content, but still always have an update at start up