Um, prior to being a state, Montana wasn't an independent nation. So if our compact with the feds is null-and-void, we're not seceding, we're just a territory, I'd think.
Update -- I may be reading into this too quickly. Near as I can tell, the argument isn't that the compact with the feds would be null-and-void, it's just that, well, it's not fair or something. It's still not a great argument.