I've cycled through all the programs that have been mentioned on this thread.
A while ago I stumbled on DoIt.IM. It is cross-platform (Win, Mac, iOS, Android) AND can be accessed on the web. Supports todo email to the inbox. Free to try, very inexpensive to buy. Very active development and fast tech support.
I've been surprised that it hasn't received attention on Connect (although I may have missed the discussion.)
(The main rough-spots from my perspective relate to UI quirks. Based on the tech support blog, I expect a number of refinements very soon.)