> So how did the INF value get in there?

By client application ?

> The field is an integer, not
> a float!?

You can check in system tables which type is used internally.
Use your field and table names in this query:

SELECT RDB$TYPE_NAME
FROM RDB$TYPES
WHERE RDB$FIELD_NAME='RDB$FIELD_TYPE' AND RDB$TYPE =
(SELECT RDB$FIELD_TYPE
FROM RDB$FIELDS
WHERE RDB$FIELD_NAME =
(SELECT RDB$FIELD_SOURCE
FROM RDB$RELATION_FIELDS
WHERE RDB$FIELD_NAME='...' AND RDB$RELATION_NAME='...'))

> How is this 'state' represented internally?

Find some document about IEEE 754 standard.

Ivan
http://www.volny.cz/iprenosil/interbase