Princeton's Office of Information Technology noticed that some iPads were having trouble with the IP addresses sent to them by the school's system of distribution. The department blocked the misbehaving units and warned students that they could have trouble. However, there never was a ban put in place. The school maintains up-to-date information on the issue here.
Meanwhile, the Cornell Daily Sun reported that Cornell University "...[has] researched the issue and have found no negative [iPad] impact at Cornell at this time." That's a quote from the university's IT director Dave Vernon. In fact, Cornell sells iPads in their bookstore.
The story at George Washington University is a bit different. Their network requires a VPN client of sorts that isn't available for the iPad. The devices aren't banned, they're just unable to access the network for the time being. According to the school's website, "We believe iPads will work on our wireless network this summer."