Once Apple said that they tested Siri in older devices but it did not work. After a hacker hacked Siri and made it work on iPhone 4, Apple declared that they would not be bringing Siri to older devices.
After the news of Siri working on iPhone 4 spread, Apple ran their own tests to see whether Siri was really compatible with iPhone 4.
Some people called it fluke. However, it seems that Siri working on iPhone 4 might be more than a fluke.
Some iPhone 3GS users practiced some jailbreaking tactics and made Siri work on iPhone 3GS. According to some users, there was a problem due differences in visual performances due to different processing speed. Siri picked voice command in a soundproof environment. This is because iPhone 3GS had only one microphone. This it is difficult for Siri to interpret voice commands in iPhone 3GS.
However, with an exception to these two problems, users claimed that Siri worked perfectly in iPhone 3GS.
Source: Apple Tell