After checking the patch we've found the following situation:
It works well on these backends: PostgreSQL 8.1, MySQL 5.0, MaxDB 7.5
It does *not* work on these backends: SQLite3, FireBird 1.5

The latter two database backends do not support statements like "foo IS TRUE". 
But since the GCondition does not know anything about the driver it is "working"
for, we cannot force it to generate 'other' SQL code then.  This would be
something to change when there are bigger changes planned in GNUe common.