I have the same setup except my iMac sports Thunderbolt. I had the same problem and never solved it, but I did find the following work-around helpful:
Place the iMac in Target Mode by going to Apple|System Preferences|Startup Disk, make sure your startup disk is highlighted, select Target Disk Mode and confirm. Connect the computers by Firewire or Thunderbolt (TDM does not support Ethernet). Start Migration Assistant on the MacBook Pro, allow it to change your system, and select the iMac's startup disk.
From there it is easy sailing - choose what you want and how to handle the user accounts and you are home and dry! Don't forget to eject all connected iMac drives before unplugging the cables.
