And only if you're using the internal microphone. I was trying to test this and couldn't reproduce it until I realized I was using headphones with a mic. Unplugged the headphones, activated Siri and the fan instantly turned off.
Is someone up for a free warranty Macbook replacement? ;)
Even back in the P4 days there were videos of people ripping Heat sinks off intel chips while running and the system throttling down and speeding back up as the heat sink is replaced.