the oracle documentation explains very nicely how to :
Use RMAN Incremental Backups to Roll Forward a Physical Standby Database (see here), what it doesn't tell however is what to do if new datafiles where created in the meantime.
At least on 10.2.0.3 the procedure didn't work if there were new datafiles created between the current_scn of the standby and the actual scn of the primary.
to check if this is the case :
select file#,name from v$datafile where creation_change# > THE_SCN_FOUND
one could think that following would work :
backup the datafiles listed above and restore them.
however the switch database to copy ; complains about following
RMAN-03002: failure of switch to copy command at 06/16/2011 21:17:53
RMAN-06571: datafile 736 does not have recoverable copy
just use catalog start with you might think, well think again it doesn't work.
what does work :
catalog datafilecopy 'datafile_names_of_the_datafiles_restored'
and the switch database to copy;