The solution is a simple iTunes AppleScript:
tell application "iTunes" to updateAllPodcasts
You can use iCal, or any other scheduling app you like, to run it when it makes the most sense for you. I use cron with the following entry:
00 5,17 * * *
osascript -e 'tell application "iTunes" to updateAllPodcasts'
So now I have podcasts downloaded and waiting for me both before I get up, and before I get home!
[robg adds: Please note that I broke the cron entry onto two lines for a narrower display here. It needs to be entered as one long line without any breaks if you wish to use it...]

