Just found this article:
http://www.ismashphone.com/2009/06/22-more-iphone-30-features-you-may-not-know.h tml
It says that the 30 second rewind "only works for media bought and downloaded from iTunes."
I can confirm this with iTunes U content. I quickly downloaded some iTunes U content on my iPhone, the 30 sec rewind option is there. Same for podcasts: all my podcasts are downloaded by iTunes, even if I added the feed manually as I didn't like the feed available in the iTunes store itself.
However, this theory doesn't work with audiobooks. Most of my audiobooks are not from iTunes, yet all have the 30 sec rewind.
It's really lame from Apple to disable this functionality for non-iTunes iTunes U content (we have couple of language course CDs, I rip them and wanted to set them as iTunes U as they are educational: I guess I need to go the audiobook route for now).