comparing subselect3.result and subselect3_jcl6.result one can see few differences.
Different EXPLAIN output is expected.
Few queries return rows in different order - which is normal too.
But one query return different results, which looks like a bug:

— r/subselect3.result 2010-11-02 10:48:15.000000000 +0100
+++ r/subselect3_jcl6.result 2010-11-02 10:48:15.000000000 +0100
@@ -1031,6 +1035,7 @@
t22.a in (select t12.a from t11, t12 where t11.a in(255,256) and t11.a = t12.a and t11.c is null) and t22.c is null order by t21.a;
a b c
256 67 NULL
+256 67 NULL
drop table t1, t11, t12, t21, t22;
create table t1(a int);
insert into t1 values (0),(1);