The developers of Spire, which was a hack to put Siri on the iPad2, have stated that Spire will not be developed further to put it onto devices with iOS 5.1 and higher.
Here's a direct quoted tweet from one of the developers (chpwn): "Spire does not (and will not for the foreseeable future) support iOS 5.1 or iOS 5.1.1, sorry." It's mainly because Apple does not host a decrypted copy of Siri for iOS 5.1.
Sorry, but no Siri on iOS 5.1.
Marilyn