A Google search has turned up 6 titles with this requirement. The ebooks come from a variety of publishers and and support a wide variety of languages (Chinese, Urdu, and Arabic). This is a rather curious bit of news, considering that iBooks is still available as v2.2. This most recent update only came out last month and it added support for iOS 6.
What's more, a French blog has reported that there is a 7th title which requires iBooks 3.0. That's one I cannot see in the Google search results. I wonder how many other titles require the new app?
Update: And here is another.
In any case, I don't see what it is about the next version of iBooks that these ebooks would need, and that's a good thing. It means there will be at least one new detail to be revealed at the Apple event 3 days from now.
We already knew that we would likely see the iPad Mini for the first time, and we already knew that it probably would cost $249. Combine these 3 details and I'm going to go out on a limb and say that Apple is making a deeper push into the education market.
The iPad costs more than most schools are happy with, and even the iPad 2 costs too much. Now with a smaller $250 iPad Mini Apple will be poised to go after a more budget-minded market segment.
P.S. See, this was why Amazon unveiled Kindle Whispercast a couple days ago. Amazon got a hint that this was coming and decided to preempt Apple.
Update: Liz Castro connected me with another ebook developer who has been making ebooks for iBooks 3.0 for the past few days. He has been tweeting his progress and one thing he's already noticed is that iBooks 3.0 appears to offer more Epub3 support. The fixed layout spec in Epub 3 requires the latest version of iBooks, only Apple hasn't released it yet.
It's not clear yet how much of Epub3 is supported in iBooks 3.0, but my guess is that Apple is going to support the complete spec. In calling it 3.0 Apple is signalling that the new version of iBooks is a major revision. It also brings the numbering in sync with Epub 3, and while that could be just a marketing trick, it might also be intended as a sign that Apple supports the newer features found in Epub3.
Curiously enough, Apple does support some Epub3 features in the current version of iBooks. But not all features were supported, so this new release is a good sign.