There are some novels (Gravity's Rainbow comes to mind) where I frequently had to look up various events and people that were mentioned in the text. I can see this being helpful, but I'd probably prefer an "annotated edition" where you could view notes written specifically for the book you were reading instead of generic Wikipedia links.
Unfortunately I haven't encountered any ebooks that handle annotations in a pleasant way that takes advantage of the technology. The few I've seen that had annotations just had hyperlinks to a different page in the ebook, which makes you lose the context. With ebooks, endnotes like these seem absurd to me; the annotation should be viewable inline on the original page.
The blog article doesn't mention it, but I've found that the footnotes have to be in the same XHTML file as the reference for this to work in iBooks. Also, iBooks for OSX doesn't display these popup footnotes yet.
Unfortunately I haven't encountered any ebooks that handle annotations in a pleasant way that takes advantage of the technology. The few I've seen that had annotations just had hyperlinks to a different page in the ebook, which makes you lose the context. With ebooks, endnotes like these seem absurd to me; the annotation should be viewable inline on the original page.