Update: note that question was asked and answered in 2012, a lot has changed in the OS since that time, including SIP, which prevents the renaming proposed here. The answer below by @cwd is now the recommended way as it doesn't need disabling of SIP...
You can rename
/System/Library/CoreServices/Captive Network Assistant.app
into
/System/Library/CoreServices/No More Captive Network Assistant.app
and you're all set. Password entering now goes via your browser of choice and can be stored using 1password or other plugins. Note that if you were already logged in while doing the renaming, it may take some time for your session to expire...