openser_test=# \d presentity
                                    Table "public.presentity"
    Column     |          Type          |                        Modifiers
---------------+------------------------+---------------------------------------------------------
 id            | integer                | not null default
nextval('presentity_id_seq'::regclass)
 username      | character varying(64)  | not null
 domain        | character varying(64)  | not null
 event         | character varying(64)  | not null
 etag          | character varying(64)  | not null
 expires       | integer                | not null
 received_time | integer                | not null
 body          | bytea                  | not null
 sender        | character varying(128) | not null
Indexes:
    "presentity_pkey" PRIMARY KEY, btree (id)
    "presentity_presentity_idx" UNIQUE, btree (username, domain, event, etag)

openser_test=# insert into presentity
(domain,username,event,etag,expires,body,received_time ) values
('192.168.150.11','116333','presence','a.1276884785.3151.1.0',1276885262,E'<?xml
version="1.0"?>\\012<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
xmlns:c="urn:ietf:params:xml:ns:pidf:cipid"
entity="116333@192.168.150.11">\\012  <tuple id="0x828d7d8">\\012   
<status>\\012      <basic>open</basic>\\012    </status>\\012 
</tuple>\\012</presence>\\012',1276884901);
ERROR:  null value in column "sender" violates not-null constraint
openser_test=#

++++++++++++++++++++++++++++++++++++++++++

mysql> describe presentity;
+---------------+------------------+------+-----+---------+----------------+
| Field         | Type             | Null | Key | Default | Extra          |
+---------------+------------------+------+-----+---------+----------------+
| id            | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| username      | varchar(64)      | NO   | MUL | NULL    |                |
| domain        | varchar(64)      | NO   |     | NULL    |                |
| event         | varchar(64)      | NO   |     | NULL    |                |
| etag          | varchar(64)      | NO   |     | NULL    |                |
| expires       | int(11)          | NO   |     | NULL    |                |
| received_time | int(11)          | NO   |     | NULL    |                |
| body          | blob             | NO   |     | NULL    |                |
| sender        | varchar(128)     | NO   |     | NULL    |                |
+---------------+------------------+------+-----+---------+----------------+
9 rows in set (0.00 sec)

mysql> insert into presentity
(domain,username,event,etag,expires,body,received_time ) values
('192.168.150.11','116333','presence','a.1276884785.3151.1.0',1276885262,'<?xml
version="1.0"?>\\012<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
xmlns:c="urn:ietf:params:xml:ns:pidf:cipid"
entity="116333@192.168.150.11">\\012  <tuple id="0x828d7d8">\\012   
<status>\\012      <basic>open</basic>\\012    </status>\\012 
</tuple>\\012</presence>\\012',1276884901);
Query OK, 1 row affected, 1 warning (0.07 sec)

As you can see PGSQL is detecting an error and does not insert this entry, but MySQL seems to ignore it and is generating an internal warning message only (but this could be influenced by the escape characters, too).

Note: the acc and missed_calls tables are okay; the not_null problems seem to occur in presence related tables only.