iGoogle (Wow, Apple made a Google service?)
iRobot (iRobot Roomba, etc. - 1990)
iBrowse (Amiga browser, 1996)
A little googling (heh) will find lots more.
Trademarks need to be chosen from non-generic words and symbols, or must be qualified by a non-generic name (e.g., "Microsoft Windows" vs. "X/Windows").
Allowing one company to co-opt a letter of the alphabet pollutes the global namespace far too much.
In this case, the "i" is a great example because it is clear that Apple does not own the "i". They do, however, on their devices, use that all over the place for their things, and if users saw it in that context it invokes "oh, this is Apple's service".
Therefore, in the restricted context of Apple's device they have a pretty good claim that if there is an "i" on something, there will be confusion. However, if listed on Google's website, or on a router, it wouldn't be; it is all about context.
"Allowing one company to co-opt a letter of the alphabet pollutes the global namespace far too much." <- This, thereby, is just a BS argument that you find on forums constantly that totally ignores how trademarks actually work.