Can you shutdown an instance from plsql?

SQL> begin
2 execute immediate 'alter database close';
3 execute immediate 'alter database dismount';
4 end;
5 /

PL/SQL procedure successfully completed.

SQL>

Shutting down archive processes
Archiving is disabled
Tue May 29 16:41:10 2007
ARCH shutting down
ARC1: Archival stopped
Tue May 29 16:41:15 2007
ARCH shutting down
ARC0: Archival stopped
Tue May 29 16:41:16 2007
Thread 1 closed at log sequence 24
Successful close of redo thread 1
Tue May 29 16:42:07 2007
Completed: alter database close
Tue May 29 16:42:07 2007
alter database dismount
Tue May 29 16:42:08 2007
SUCCESS: diskgroup ASM1 was dismounted
Tue May 29 16:42:09 2007
SUCCESS: diskgroup ASM2 was dismounted
SUCCESS: diskgroup ASM3 was dismounted
Tue May 29 16:42:09 2007
Completed: alter database dismount
(END)

It doesn't look like you can actually shutdown the processes, though, without external code.

Also, you can't re-mount a database shutdown in PLSQL, as you will get an

ORA-00750: database has been previously mounted and dismounted.