While our User Experience team has not defined this down to the last possible aspect we do provide some reach into these different standards. More specifically we refer to these as Messages and Contextual Help.
The "i" informational symbol is currently being used as part of our set of message elements (See image below). Because our User Experience team works with both internal and external facing applications we have developed for a wide range of possible messages. Depending on the content, developers are allowed to choice the message style that makes the most sense. For the most part the "i" informational message is the most used as it best represents generic messages.
These message elements are specifically made for informational purposes only. Meaning that something is important; and the users should be informed. The different styling is to better emphasized specific message types. We find in some cases using the "i" symbol is to generic and these other message styles better reflect the type of content needing to be conveyed.
For the "?" icon we use it as reference for contextual help (See image below). Our contextual help for the moment is tied to the Qtips 2 jQuery plugin. This way users can reveal the contextual help as they need it. This is most commonly used on forms but developers are not limited to linking the icon to another web page.
The two icon defiantly have different meaning. The "?" icon has been the long believed defacto standard for help. The "i" informational symbol to my knowledge is slightly newer but has also become a said standard. I would hesitate from using any other icon symbol to reflect either.