SQL Server 2005 has a new backup option, that allows you to create log and database backups without breaking the chain. Previously, taking an out-of-schedule database backup would ruin your differentials, you have broken the chain, likewise for log backups, your LSN's would be out of sync.
However, now if you need to create an adhoc database or log backup you can use the COPY_ONLY option, this will not screw with your LSN's or chain and your logshipping and differentials will continue as normal.
BACKUP DATABASE Airline TO DISK = 'E:\Airline.BAK WITH COPY_ONLY
or
BACKUP LOG Airline TO DISK = 'E:\Airline_20080102.TRN WITH COPY_ONLY