I had the same problem using redsn0w on the Mac, the iphone behaved identical. What worked for me was to take the phone out of the cradle and directly back in. Be careful though to only do that at this phase of the JB process, i.e. wait and make sure that nothing is going on on your phone and redsn0w is waiting for a response from the phone.
See here for details:
http://www.hackint0sh.org/forum/f205/77965.htm

Hope this helps

Clownkiller