You consider downloading the deb file for spire and using the plist from that? I'd be curious if that works.....or get a hold of the spite deb and use that since it does work on 5.1 tethered.
I didn't test the Spire files + 5.1.1 files yet but I need someone to test something.
If something is willing to try it, do the following:
1. Download the SiriPort .deb I posted the link to a few posts back ( it contains the Siri 5.1.1 files, which I can't upload at the moment)
2. Extract the .deb on your computer (I did it using uniextract:
http://www.softpedia.com/get/Compression-tools/Universal-Extractor.shtml )
3. Delete the folders var and etc inside the extracted folder
4. Delete the K48AP.plist, N81AP.plist, N88AP.plist, N90AP.plist, N92AP.plist, N94AP.plist from System\Library\CoreServices\SpringBoard.app on the extracted folder
5. Here the testing begins... Copy the extracted folder to your iPhone's root
6. Go to \System\Library\CoreServices\SpringBoard.app on your iPhone and locate your AP.plist file ( N90AP.plist for AT&T (GSM) iPhone 4, N92AP.plist for Verizon (CDMA) iPhone 4, N88AP.plist for iPhone 3GS) and add <key>assistant</key>
<true/>
to it.
7. Save the AP.plist file, Reboot and comment here on what happened.
One of the following is supposed to happen:
1. Your iPhone reboots succesfully and Siri will appear on your settings, after you activate it in Settings, Try to talk to Siri and after you get a response the file /var/mobile/Library/Preferences/com.apple.assistant.plist should be created and you can set your proxy in the file.
2. Your iPhone will fail to reboot and will get stuck on the Apple logo. In this situation enter DFU Mode, Restore and rejailbreak
Remember to post what happened after the reboot if you are willing to test this.