I am typing this at work without access to my setup so please forgive me if this doesn't work perfectly as this is off the top of my head.
Here is how I did it between Mac OSX and XP. I have not tried this for podcasts, so it may not work. I see no reason for podcasts to be any different, however, since they are just files downloaded to the computer.
This works if you have either a seperate FAT32 partition, an external FAT32 drive, or MacDrive installed on your windows partition.
The reason you want one of the 3 options listed above is because it will give you a place to store your music/movies/podcasts in a place that both operating systems can access. If you have an external hard drive then the easiest (read free) option is to format it as FAT32 and copy all of your media to this drive.
After you have copied all of your media to the drive you just need to point both copies of iTunes to the drive/folder all you media is now stored in and you should be good to go.
This way when you download a song or podcast on either OS it will be stored in a common location and the other OS will see it when you open iTunes. Saving you the hassle of having to redownlad everything.
This does have the unfortunate side affect of using 2 of your available 5 PC activations. But this only matters it the podcasts or songs are from the iTunes store. If they are ripped from a cd or downloaded from another site they are not tied to your iTUnes store account and you have no worries.
I hope this helps.
~CrimsonEva~
P.S. First post here. I hope it was a good one.