Developer Eric Day has released Siri0us, a package for jailbroken iOS devices that enables Siri dictation functionality.
Siri dictation http://www.if0rce.com/siri0us/ Please retweet, I will release it when getting enough retweets. 
” It’s alive now!
To install Siri on your device you must jailbreak and install the Siri0us package from thehttp://apt.if0rce.com Cydia repository.
Initial reports reveal that Siri0us works on the iPhone 4, iPhone 3GS, and iPod touch 4. We will be posting instructions soon. Stay tuned!
[via Martin]
Update:
The if0rce repo is getting slammed so you may have trouble adding it to Cydia at this time.
