Null values where they should not be...HELP Please!!

I am using postgreSQL v. 7.0.2 on RH Linux 6.2 on the server with VB6
Application accessing the DB through postgrSQL ODBC driver v. 6.50.

We have been running one application for a few months now and everything
seemed to be fine and dandy. We are now exploring PG 7.1 and in the process
of dumping the data for upload to 7.1 for testing, we found something very
strange.

- One Table has its default value as false, and has a constraint that it
can't b null, still there are a lot of values that are NULL.

- A serial field supposed to be "not null" also has a lot of nulls.

- boolean fields with default values specified have nulls.

Assuming even that the application is trying to store null where it should
not, PG should raise an error, instead of accepting the same. Anyone
encountered this before? Where should I look for a fix to this particular
problem while we are testing PG 7.1

My thanks in Advance