Hi,
Siri does have its uses.
Also their are current threads about siri and how to have it on a 3gs or 4.
1st you will need to jailbreak your iPhone.
Then download spire through cydia.
here comes the choices,
You could opt for a free siri proxy but i wouldn't recommend it for reasons you will find out when doing some research.
I opted for imzdl's service for a year which costs $20 or £12 in my money.
They send you an e mail which you open on your iPhone with a couple of links within the e mail. 1st link is to download their certificate. 2nd link is to down load their 8 servers. Then you choose the nearest to you and simply copy and paste the proxy into the spire host in settings or just simply tap in the numbers yourself.
Enable siri in your general settings and your away.
Some useful tweaks to compliment siri are
hands free control
siri launch