People will do anything to get Siri onto non-iPhone 4S devices, and we saw that with the tweak Siri0us (which was ultimately removed). This time around, H1Siri has emerged by a group of hackers in China called the CD-Dev Team. This hack brings full-fledged Siri to your iPhone 4 or iPod touch 4th generation on iOS 5.0 or 5.0.1.
The hack, available on a custom repo in Cydia has been deemed illegal and potentially dangerous if installed, as noted by @chpwn. The copyrighted binaries involved are taken from the iPhone 4S, plus all your personal data from email, SMS, Calendar, Contacts, etc will be exposed to third party servers.
iDB posts a hands-on video of the hack, via the custom Cydia repo: http://www.iphone3gsystem.fr/cydia/
We do not encourage piracy or breaking of any copyright laws. This story is an example of what people will do to get Siri on their iPhone 4 or iPod touch 4th gen, at any cost.