Did you ever find anything? Aside from rudimentary 1-way handling (you can email yourself something that becomes a task) I found one for Outlook and two others: ActiveInbox that turns gmail into something GTD (and is working pretty well for me right now despite how much I despise gmail) and IQTell which looks like it works with any email system and is more of a GTD tool overall. IQTell looks promising although it has a steep learning curve. Works with iPhone too.
Amazing how much more time it's taking me to find the right tools to use than it is to figure out how GTD can ultimately work for me...