You'd still have trouble explaining that to a judge. The hotel has a reasonable expectation that if the traffic says it is coming from an iPad, it's actually coming from an iPad, and you don't have any non-infringing excuses to be using an iPad browser on your laptop.
If they advertised the plan as an ipad plan, there would be a point. But if the plan is advertised as all-purpose, but only offered to certain user agents, i don't think there's any legal issue.
