It's not that apple is being mean and not letting you have it, it is that Siri is very processor intensive, the ipod touch fourth generation runs off of the A4 processor.
the iphone 4S runns of of the A5 processor, which is many times more powerful, (and there are a couple of voice recognition chips that have been updated that Siri needs that only appear in the iPhone 4S as of yet).
If you can't get/don't want an iPhone 4S, and want siri that badly, buy the updated ipod touch when it comes out (likely in August) or if you have an iphone 4 (and possibly an iPod touch fourth generation, I have not tested that device with this fix) and don't mind voiding your warranty and frequent access to an iPhone 4S. You can in fact jailbrake the device where you want siri and run a couple of applications on it, then copy a file you need from the 4S and run Siri on the other device
But before you put in all the work do know that Siri is still a beta product and fallable and not perfect, I love her to pieces, but unless you have a specific reason that you need her on your device the work to put her on almost out weighs what she can do for you