I have a similar problem: I connect to an unsecured wireless router provided by my residential building. That works fine, and in order to access the Internet, I enter my authentication details in the first browser window that comes up. However, I need to change my authentication details to another account; but, I get logged in automatically when I open my browser and never see the login browser window. I don't know if the login data is being sent by Safari or if my MAC is recognized and that causes the auto-login. Any suggestions on how to keep from being automatically logged in? I did try the "forget this network", but that doesn't do anything because that just applies to the wireless access point that I'm connecting to, I think. Stumped, I am, Iam.
UPDATE: Of course after I submitted this question, I found the answer. I'm not exactly sure which action cleared the problem, but after someone suggested that I go to Settings >> Safari and clear my cookies, etc. and in Wi-Fi use "forget network", turning off "ask to join", powering off, then powering on and trying again, the browser login screen appeared! I think it was probably the cookies, but not positive. Anyway, fixed!
