I'm using a xv6900 on verizon and I haven't been able to sign in either. I wasn't able to get in on the last public beta but I did sign-up a couple weeks after that one closed, so I didn't create a brand new account but I still was not able to sign in this mourning after downloading the new beta (.80).
Skyfire has released an announcement:
http://support.skyfire.com/forums/in...showtopic=1597
