Same issue here:
tried to install winterboard, got a message about a failure when trying to overwrite the mobilesubstrate..when I clicked ok the same thing happened as mad_llib described. Scared the shh out of me at first until I rebooted and everything was fine
check out this thread:
http://www.modmyi.com/forums/winterb...nterboard.html
this is what happens when the winterboard install is successfull, just a blank screen when I try to access it. It looks like the only way to fix this is to repwn and keep trying until winterboard starts to work!
