just pondering this today and found out 63 characters is the limit for a domain name

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.com

http://www.oreillynet.com/onlamp/blog/2005/06/the_worlds_longest_domain_name.html

http://3.141592653589793238462643383279502884197169399375105820974944592.com/