I was trying to use an INSERT ... RETURNING as a subselect in 8.2. Is
that possible?

jdavis=# create table foo(i int);
CREATE TABLE
jdavis=# insert into foo(i) values(1) returning i;
i
---
1
(1 row)

INSERT 0 1
jdavis=# select * from (insert into foo(i) values(1) returning i) t;
ERROR: syntax error at or near "into"
LINE 1: select * from (insert into foo(i) values(1) returning i) t;
^

If not, is there a reason it shouldn't be allowed, or is that a possible
feature for 8.3?

Also, why no GROUP BY or aggregate functions?

I was interested in using the RETURNING clause in place of using
PQcmdTuples() to get information about what was inserted. I don't think
there's any way for a function to modify what is returned by
PQcmdTuples, right?

Regards,
Jeff Davis