Assuming that multipathing works as designed, there will be a delay as the OS attempts to use the existing path, the path will failover and the transaction will complete using the new path. If the failover doesn't take place within the specified timeout period, then the system will normally crash. My experience with Oracle on Soalris is that the system appears to be alive but completely hung and you need to reboot.
I've seen backups run without problems during HBA failovers and database transactions complete - with a short delay.
If you are on good terms with your SAN team, have them disable the HBA port at the switch or SAN and you can verify that failover works properly. (Note- HBA FC SAN)
Mike