plpgsql help - nested loops

I'm trying to compare the fields of two loop record variables. The loops
are nested, the compare is done inside the inner one. If given this:

raise notice ''sloop.serialnumber=%, rloop.serialnumber=%'',
sloop.serialnumber, rloop.serialnumber;

if sloop.serialnumber=rloop.serialnumber then
raise notice '' SN match'';
else
raise notice '' SN mis-match'';
end if;

the output is

NOTICE: sloop.serialnumber=<NULL>, rloop.serialnumber=<NULL>
NOTICE: SN mis-match

However, if you change the comparison to

if sloop.serialnumber=rloop.serialnumber or
(sloop.serialnumber=null and rloop.serialnumber=null) then
raise notice '' SN match'';
else
raise notice '' SN mis-match'';
end if;

everything is fine.

Question: does null=null evaluate to true, false, or null? If in psql you
do SELECT null=null; it returns true. Am I missing something? Attached
is the full trigger code..

Thanks,
-Cedar