We are also experiencing the same problem on IE7 with DNN 4.5.3 on 2003.
The user doesn't remember using the Remember Login checkbox, but that doesn't necessarily mean that he didn't. I just say that in case it isn't just when using Remember Login.
I used GotoMeeting to verify that the correct entries are being made, if you enter invalid login, you get warning, if valid, it just returns to home page in state that is not logged in.