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.

Johannes, can you please test if this patch works with all backends?

Thanks, Reinhard

the attached patch implements IS TRUE, IS NOT TRUE, IS FALSE, IS NOT FALSE sql
conditions. it simplifies the writing of complex conditions:
["true", ["field", "field_name"]]
instead of ["eq", ["field", "field_name"], ["const", True]]

needs review and needs testing for db different than postgresql.