Converting standard_conforming_strings from off to on when importing Postgres dump
Testing with a 8.1 instance, it doesn't look that there is a problem. See these 3 different examples:

Create data

 SHOW standard_conforming_strings ;
 standard_conforming_strings 
 -----------------------------
  off

CREATE TABLE bla(t text);
INSERT INTO bla VALUES('abc''def');
INSERT INTO bla VALUES(E'abc\\def');


1. Dump of 8.1 data with 8.1 pg_dump

$ pg81/bin/pg_dump db


this produces (after removing the irrelevant stuff):


COPY bla (t) FROM stdin;
abc'def
abc\\def
\.


Here standard_conforming_strings does not matter because the COPY format is independant from it. When this COPY is executed, it will make no difference whether it's on or off.

2. Dump of 8.1 data with 8.1 pg_dump with --inserts option

$ pg81/bin/pg_dump --inserts db


So COPY is not used.
The dump now contains, for the interesting part:

INSERT INTO bla VALUES ('abc''def');
INSERT INTO bla VALUES (E'abc\\def');


Since the E'...' syntax is used, standard_conforming_strings is again irrelevant. These strings will be interpreted correctly,  whether the setting is on or off, by any version of postgres>=8.1

3.Dump of 8.1 data with 9.1 pg_dump with --inserts option

$ pg91/bin/pg_dump --inserts db


Now the dump happens to contain this, somewhere at the start

SET standard_conforming_strings = off;


and then in the data section:

INSERT INTO bla VALUES ('abc''def');
INSERT INTO bla VALUES ('abc\\def');


So again it's being taken care of automatically, albeit differently than with 8.1's pg_dump, but this dump is going to be reloadable directly in any version of postgres>=8.1, without tweaking anything.