Yes, the internet is blocked in the Cornell Law lecture halls. Some tech-savvy people might be able to figure out a way around it though, because I've seen people IMming in class. It's not that there is no wireless signal; they just scramble it in the room or something. Maybe you can use bluetooth to connect to the internet? I have no idea.
It's actually nice not to have internet access though. Being forced to pay attention in class and to take notes isn't a bad thing... And a lot of professors let you use your notes on the finals, so you really wouldn't be doing yourself any favors by being online instead of taking notes. And you're in class for like 4 hours out of a day, max. It's not the end of the world.
Some of the smaller classrooms are not blocked though, so people can use internet in those rooms. The wireless signal in those smaller rooms work much better for Macs than PCs though.
HTH
-Cornell 1L