No redirection to checkout page after signup process - using link "wp-signup.php?Premium=1&quot

Hi guys,
I disabled the option "Show Option On Signup" and using the signup link "wp-signup.php?Premium=1" for my Premium-level to force and hide the premium option during signup. This feature used to work with a previous version of Pro Sites. I updated to the current Version 3.0.9. and now it's not working anymore. I also tried to use the new link "wp-signup.php?domain+Premium=1". The hidden form field <input type="hidden" value="1" name="psts_signed_up_override"> is not showing up during the signup process.
I am running WPMU 3.3.1.
Any idea? Thanks in advance