I've some oracle calls that i'm porting. Today I discovered this code which appears like "procedural" language, however is not declared inside a function or anything... My real question is: Can postgres handle this within this form? What form performs this have to be in?

DECLARE
BEGIN
   IF :start_time IS NULL OR
      :start_date IS NULL OR
      :end_time IS NULL OR
      :end_date IS NULL  THEN
         INSERT INTO ARPSPACE_AVAILABILITY
            (ARP_ARPSPACE_NM, ASA_TIME_ID, ASA_START_DT, ASA_END_DT)
         SELECT :arpspace_name,
                 1,
                 ASP.ASP_START_DT,
                 ASP.ASP_STOP_DT
           FROM ASP 
          WHERE EXISTS
             (SELECT ARP.ARP_ARPSPACE_NM
                FROM ARPSPACE ARP
               WHERE ARP.ARP_ARPSPACE_NM = :arpspace_name);
   END IF;
END;

No, not without proclaiming a function. In version 9. (soon in Beta) this is possible: http://developer.postgresql.org/pgdocs/postgres/sql-do.html

Short response is yes, if you are able to declare a function. See plpgsql in the manual for particulars.

Why don't you do that (presuming :labels are ready query parameters)?

     INSERT INTO ARPSPACE_AVAILABILITY
        (ARP_ARPSPACE_NM, ASA_TIME_ID, ASA_START_DT, ASA_END_DT)
     SELECT :arpspace_name,
             1,
             ASP.ASP_START_DT,
             ASP.ASP_STOP_DT
       FROM ASP 
      WHERE EXISTS
         (SELECT ARP.ARP_ARPSPACE_NM
            FROM ARPSPACE ARP
           WHERE ARP.ARP_ARPSPACE_NM = :arpspace_name)
      AND  (:start_time IS NULL OR
            :start_date IS NULL OR
            :end_time IS NULL OR
            :end_date IS NULL);