It's not just a problem with the 3G. I've got a 3GS running OS4, with iBooks 1.1. When I open iBooks I get the shelf or list view, but without the Books|PDFs button. The header has an Edit button on the left, "iBooks" in the middle, and a Store button on the right.
When connected to iTunes, the 3GS shows up and the Books category shows up under the 3GS. When I click on Books, I see (greyed out, as usual), the PDFs that I have loaded into iTunes. The PDFs are on the 3GS, but the app is not display the controls properly, very much as though it has not been updated to 1.1.
I have deleted the app from the 3GS and iTunes, deleted the PDF files I had imported, re-downloaded the app and synced the 3GS, added the PDFs and synced again, etc., but the app is not working properly.