I just went to the Madonna.com site to sign up and it seems that there is a cut off for new members to join and have access to presale tickets. In other words, if you join now, you will not have access to upcoming presales.
"PLEASE NOTE: the cut off to register for a fan club membership that allows you presale access to concert tickets was 11:59 pm EST on April 30th 2008. "
I figured this would be good information to pass on in case there is anyone who was thinking about joining the site now for presales and did not read this.
Anyway, I'm just looking out and hope this helps.