Probably. Take note of the fact that OS X editions don't use a serial number. You can very easily share them with friends and family and online. Same goes for iWork, however some of the more expensive software does use SN.
If you already bought an iphone/mac/ipad (in the future) that has Siri, then I don't think apple will care much if you use siri on other devices. However what is really useful with siri is where it talks to the os layer and other applications. That kind of integration isn't all that easy to do.
So if someone writes an app with the integration to the os and apps (calendar, sms, phone, phonebook...) and decide to use Siri (illegally), then I think they deserve a medal or soemthing for their hard work for porting the siri front end to another platform.
