I've said it before, I'll say it again. Someone should create a standard way to build inbox addons... AND IT SHOULD NOT BE ME.
"Notably, Xobni is also working with third party developers to let them build applications with Xobni. By allowing third party integration for its APIs, Xobni becomes a trojan horse for those third parties to access Outlook integration through a plugin. Microsoft Outlook doesn’t offer a friendly set of APIs for people to plug into Outlook, and so Xobni hopes to become the place developers come to for such access"
Doesn't seem like a large stretch of the imagination to think they will do this for webmail as well, especially with the "leak" of their Yahoo mail integration.