http://technet.microsoft.com/en-us/library/cc784390.aspx
I ran into this issue when using SysPrep and Virtual Center tempaltes. The username for the Service Account was longer that 20 character and no matter how hard I tried vCenter would not add the newly created machine to the domain. When I tried with just the first 20 characters of the user name account it worked like a charm!
The maximum length of a domain name is 15 characters, and the maximum length of a user name is 20 characters. Adding a character for the backslash (“\”), the field should allow a total of 36 characters.
Active Directory Object Names
http://technet.microsoft.com/en-us/library/cc776019.aspx
NET USER command
http://support.microsoft.com/kb/251394
http://www.ecst.csuchico.edu/~sim/546/notes/w2003NOTE4.htm
