I dunno, I have mixed feelings about opening it up to 3rd parties... I think it would be difficult and confusing if not implemented correctly. Although that's not to say there isn't a smart way to do it. Maybe it would be similar to multitasking, where there are a few specific API's that can be tied into. Like a restaurant API, movies API, maps API, etc. Then the 3rd party solution could simply replace Apple's. But when I think about it, that seems about as likely as Apple letting other browsers become the default...
I kind of feel like Apple will just keep expanding Siri internally, at least for a while. or possibly add very simple tasks for 3rd party apps on the same level as tweeting/fb status.