In his most recent post, Ken Segall shares his thoughts on Apple's "i" as well as some history such as this bit:
The truth is, the idea of moving past the i had come up at various times inside Apple. In fact, I had a conversation with Steve Jobs on this very topic way back in 2006.
On a related note, I've read mixed opinions on the name iPhone Plus. Personally, I think it works. And although it's likely unrelated, it's interesting to note that "plus" isn't a new naming convention for Apple. In 1979 it was used in naming the Apple II Plus and of course there's currently AppleCare+.