[detaro]

But if you need to sign updates, a third party CAN'T insert new code without it going through the site owner or a trusted party (e.g. new jQuery versions signed by the jQuery project). And it would seem desirable to be able to roll out a security fix without having to touch every single page that includes it (and the potential cache issues discussed above)

[jsprogrammer]

You can create similar semantics. The only difference is in who controls what code goes to your users.

I'd consider jQuery a third party. If only signatures are checked (and not content), then trusted third parties can push whatever code they'd like to your users.