I am having similar issues. I started a thread about it earlier this morning, no solutions yet, but:
I run an IT department for a college that utilizes a captive portal. It seems that the issue is with the iOS6 update. We require everyone to authenticate via our captive portal to use anything other than e-mail. We allow e-mail to pass without authentication. This is already causing A LOT of problems for me as i have half of our 2000 wi-fi user base using iDevices. With any other iOS version my users could connect to our network and receive e-mail without authentication. Something needs to be done about this....am waiting for Apple to call me.