newbieish exception/multilevel transactions

Hi,

I was wondering, is it a possibility/projected to have nested transactions
in postgres? Would MVCC support that?

What I would like is following:
begin tran a;
do some work;
begin tran b;
do some other work
rollback b;
<at this point, only changes by transaction b are rolled back>
...
commit a;

Alternatively, is there a way to trap an exception in plpgsql? [I would
assume that depends on SPI ability to trap exceptions...]

These features are really needed for people who do serious
database-backend scripting, to compete with sybase (who has multilevel
transactions) and oracle (who has very good exception handling).

Thank you

-alex