Our main iTunes box had flaky RAM -a heck of a problem to debug by itself- so it used to BSOD quite a lot during the past couple of months. One of the most common side effects was that the iTunes Podcast playlist would appear empty every now and then. Luckily this is addressed in the Apple Knowledge Base and the solution amounts to using File|Add Folder to Library to add the {iTunes Music folder}/podcast folder to the library. Then you will have to re-activate your subscriptions by pressing the “Subscribe” button for all your favorite podcasts.