I've been having the same issue (and more) but now it is fixed.
Basically it seems the new iMac is too fast for its own good, that flashing black line appears after the message "press a key to boot from cd" but that message goes so fast you can't see it. What you want to do it when booting keep pressing keys very quickly to make sure one is pressed during the short moment in time the iMac waits for a key.
As reference, a further issue I had was a message along the lines of "invalid disk" wasn't that, but similar. This happened after the does based windows install (initial blue screen part before it reboots into a more graphical install). This can happen if you select the "leave file system intact" option rather than formating (not quick) to NTFS. From what I've read I'm lead to believe that this is only relevant if you make your bootcamp partition more than 32Gb (as if its 32Gb or less you can leave it as FAT32 which is better really as you can write to it from OSX)