The ISPConfig manual states that database names must not be longer than 16 characters, implying that MySQL does not support longer database names. Although I don't have access to manuals of MySQL 4.x and older, it seems this 16 character limit concerns those older versions only. In the MySQL 5.x manuals one can see that the limit now is 64 characters (see MySQL 5 Reference Manual - Schema Object Names
Nice to know for users such as myself, who prfere database names starting with a readable [CLIENTNAME] instead of a cryptic [CLIENTID].
Beware though! User names can still be no longer than 16 characters.