You can certainly replant a round if it is healthy. A large round should produce a really big divided bulb next season.
Gardeneer also had a thread about this problem last year, which led me to believe it only happens with Turbans and that it didn't affect the crop. So I am curious Tom, to hear that you had this problem too! What variety was affected?
Here is the other thread about this:
http://www.tomatoville.com/showthread.php?t=47380