Is this fast:
select ... from .. where lower(username) = lower(?)
This may or may not be fast - it depends on how the query engine runs, and whether the optimizer can use an index when you apply a function on a column. It may work fine with Postgres, but I know that Sybase will not generate a good query plan with such a query.
BTW - If you use "LIKE ..." you can normally escape any potential wildcard characters yourself. Sybase uses square brackets to do the escaping (and can use alternate escape characters as well), so that you could write:
SELECT ... FROM ... where foo like ? ESCAPE '\'
and then pass "foo\_" as the search parameter and not get wildcard expansion on the underscore. Check the Postgres docs for similar functionality.