Yes, add in your postgresql.conf and restart (or just reload) db server:
log_statement = 'ddl'
That works with createuser, dropuser wrappers, CREATE/DROP ROLE/USER and ALTER ROLE/USER statements as I checked:
2011-05-23 14:40:45 CEST STATEMENT: DROP ROLE thomas;
2011-05-23 14:40:52 CEST LOG: statement: CREATE ROLE greg LOGIN CREATEDB;
2011-05-23 14:42:36 CEST LOG: statement: ALTER ROLE greg NOCREATEDB;
log_statement (enum) - Controls which
SQL statements are logged. Valid
values are none (off), ddl, mod, and
all (all statements). ddl logs all
data definition statements, such as
CREATE, ALTER, and DROP statements.
There is one drawback, that you have another DDL statements logged as well.