Apple has sort of tolerated the jailbreak community for quite some time. However, if they flat out steal apple's code, I think we'll see Apple take it to another level.
Also, since Siri requires contact with remote servers for most of its big time features, Apple could pretty easily build in a way of detecting whether or not you're using an authorized version of iOS5, and remotely brick your phone if it's found you're using illegitimate hardware with Siri. Not saying they would do that, but they very likely could. Especially if it's revealed that people are actually ripping off Siri's source code.
Edit: not saying I think Apple should do this, but they very well could. What would people do if they did? Sue Apple? For improper usage of stolen IP?