iBooks on iPhone doesnt recognized synced PDFs

I hope someone can help me with this very weird problem. I imported some PDFs into the iTunes library and marked them as books. They show up in the Books section of iTunes. I connected my iPhone 3GS (iOS 4.3) and set the option to sync All Books in the Books tab. After syncing, when my iPhone is still connected to the computer and iTunes, the PDFs appear in the iPhone window of my iTunes. So it appears as if they are in the iPhone.
Yet, when I go to iBooks on my iPhone later, I cannot find any of those PDFs. Searching the iPhone doesnt bring up any of the filenames of those PDFs, or author names or anything.
I am probably doing something stupid here, but could someone please point out my mistake?
iOS 4