Getting your Mac to play a playlist on demand at a certain time isn't hard at all (Awaken will do it easily, or Alarm Clock 2 will do it as donationware), but the tough part is getting your podcasts rounded up into a playlist so you can hear all the new shows all in one go. What I wanted to do was create a playlist that contained one of each of the daily podcast shows that iTunes had downloaded for me most recently, and to have that playlist update as the podcasts did, so that every day I had a playlist full of new show episodes set to play one after another.
Easier said than done (and it's not even that easy to say). But I got it done, just by manipulating a few iTunes smart playlists. Here's my short tutorial how.
I have to say first of all that this idea isn't mine-- it came from this post by Cosmonut over on AppleInsider's forums (I found it during one of my many Google searches while trying to figure this out). But here's my explanation of how to do it-- I wasn't able to find an easier way. Basically, what you'll do is make a bunch of different smart playlists for each podcast you want to use, with one show each, and then automatically combine them into one master playlist that will update itself every time a new podcast shows up.
First of all, obviously, you have to subscribe to the podcasts you want to download. I rounded up a few of my favorite daily podcasts, with the plan of putting them all together so I'd have something to listen to as I woke up in the morning.
Next, what you'll want to do is create a Smart Playlist for each of the podcasts you'll want to include in the master playlist. Create a smart playlist, and then set Podcast to "true," (see below for the settings) and change the Album to whatever your playlist's Album is (I had to "Get Info," or ⌘-I, on a few of the shows to figure out what they used for an Album identifier).
Finally (and this is the key), you'll want to check "Limit to," and limit the playlist to one item selected by "most recently added." That will give you a playlist with only one item in it: the most recent show iTunes has downloaded for you.
Pretty convoluted for iTunes (although podcast playlist support has never been great anyway). But it does exactly what I wanted to do: put all the latest daily episodes of my selected playlists into one list ready for me to play automatically each morning as I wake up and brew the coffee.
Update: Hessi in the comments makes a good point-- the folder you put the playlists in itself should work as a playlist for the purposes of playing the podcast, so you may not have to create a master playlist at all. But I'm not sure that folders work exactly the same as playlists in all instances (on iPods, during syncing, etc.), so it might be worth creating a separate master playlist anyway.