Ah, okay. Sorry, I wasn't clear. I do know that the app is separate from the OS.
For some reason I had assumed that along with a shiny new OS, Apple would have made some upgrades to the iBooks app. I was puzzled that no one had mentioned any changes. Now I see that it's because there weren't any!
So perhaps the next likely event for an iBooks update will be when iOS 4.2 comes out, which will be sometime in November, apparently. Not too long now!