Does psql use nested transactions?

In 8.0 beta 1 I just tried:

psql template1
> begin;
> select * from pg_class;
... got stuff ...
> select * from aaa;
ERROR: relation "zzz" does not exist
> select * from pg_class;
ERROR: current transaction is aborted....

Should psql run every statement in a nested transaction and only rollback
that TX? Or is that not possible/desirable. If possible, this would be a
*great* feature for those of use prone to mis-spellings.

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