At the centerpiece of Apple’s iPhone 4S launch is Siri, a voice-activated virtual assistant. Want to know if you should bring an umbrella with you tomorrow? Just ask.

Siri’s domain functionality is limited (for now), but thought leaders in the digital space are already filling in the blanks: Google’s search advertising revenue is at serious risk.

Why is that? When a user asks a question, Google responds with a pile of blue links. They’re a middle man — they don’t give you what you’re looking for, they tell you where you can find it. Siri, on the other hand, provides the answer. That’s what people want.

In other words, Siri’s intelligence promotes transactional efficiency.

Before Siri was acquired by Apple, the company planned to target the fat end of transactions, the most common things people are looking for. Its presence on the iPhone is only going to amplify that effect. Google’s going to be cut out of its own profit center.

At this point, I could talk about Apple’s genius. They’ve made all the right moves over the last decade, ten years of incredible growth. But I want to look more closely at Google.

Success is cyclical. Google has known all along that its search engine dominance would eventually fall. Not to a direct competitor, but to a player offering even greater transactional efficiency. That’s why they’ve fought like mad to diversify their sources of revenue.

It is odd that Google was not a first-mover for voice interface. (Yes, I know they process spoken word into text, but that’s just not the same.) It’s like Google didn’t want to drop a bomb on their core business. I’m going to assume they’ve been working on a Siri-like product for a while and prepping to release it in the near future. But I’m not holding my breath. I’ve lost confidence in Google’s vision. What was once unthinkable is now a distinct possibility — Google’s lack of foresight could send it into free fall.

Siri also has implications for the direction of mobile. We’ve assumed for some time that mobile’s future is focused on touchscreens and fewer pixels. It’s obvious, today, that voice will play a significant role, particularly in common transactions.

Everything is changing again.