Hi Doron,
yes, if you have a standalone server with multiple instances installed, you just schedule the backup with the hostname of the SQL server, without needing to mention the names of the instances:
new-backup –svr 'Host1' ...
if you prefer the SMSQL GUI, you need to click the top level check box:
instances which you install later (if configured within the same lun's of the other instances and within the correct configuration), they will be backed up too, because the new-backup command will not include a specific instance name and will pick up all instances currently installed.
hope that helps,
Domenico Di Mauro.