I'm pretty sure there's already a rule regarding patenting obvious ideas. It begs the question, if something is so obvious, why hasnt somebody already created those things by the year 2011?
Bear in mind it's not only the feature itself that's patentable but the context it's used in, so the fact that voice activated assistants appeared in star trek years ago does not mean people can't patent them for smartphones, or that Microsoft can't patent them for being in Kinect.
Don't get me wrong, I think the lawsuit is a ludicrous one, but mainly because of how Apple is trying to resolve it. If somebody owns a patent, it's their right to seek payment for the use of it by competitors, but asking for an outright ban is anti competitive and they're trying to become the monopoly that Microsoft was back in the day.
IMO it's not the patents that are the issues, it's Apple's attitude towards resolving patent conflicts.
As a US Patent attorney, I can verify that there is a rule against obviousness. However, there are a few key thinks most people don't understand about patents:
1. patent is granted several years after it is applied for, but obviousness has to be proven from before it was applied for. So three years later when it is granted, the idea may seem obvious even if it wasn't when the patent was filed.
2. Obviousness must be proven with evidence. You can't just say "that is obvious." Documentation of the claimed features existence prior to filing the application must be shown. And unfortunately, computer people don't document very well.
3. Only the patent's claims matter. The title and the description do not give any patent protection, only the claims. So the patent title might be something broad like "Voice recognition software," but the claims might contain very specific features and/or algorithms for implementing a particular type of voice recognition software. Such a patent wouldn't cover all voice recognition, just one implementation but the internet seems to think it covers all voice recognition.
It should be noted that the above example is hypothetical. I haven't looked at Apple's Siri patents so I have no idea what they claim and how broad or narrow they are.