Apple has posted a couple of job postings for iOS software engineers, and they describe work needed on UI elements for Siri, along with additional language support and a new API that could indicate Apple may be opening up the feature to 3rd party app developers.
This is a broad-ranging task - we take every application that Siri interacts with, distill it down to fundamentals, and implement that application's UI in a theme fitting with Siri. Consider it an entire miniature OS within the OS, and you get a good idea of the scope!
Siri launched with the iPhone 4S with limited language and region support and beyond built-in apps, currently only interfaces with Apple's own Find my Friends App Store app. Even built-in app support is inconsistent, for example SMS/iMessages can be read via text-to-speech but not email messages.
More engineers might also be necessary if Apple wants to expand the service to other devices, like the iPad or rumored Apple television set.
Source: Apple Insider