There are two companies with iSCSI initiators for OS X. Studio Network Solutions and Atto. If you do not use the products from one of these companies, you will not be able to use iSCSI in OS X. I am partial to SNS, as I work for them ;). However I have seen both initiators working with FCP. One big difference in the two initiators is how they can be implemented with hardware. The SNS solution is geared more towards people buying complete iSCSI SAN solutions. It comes in packages starting at 2 users (with all iSCSI software and hardware for the SAN including SANmp, SNS’s volume sharing software). It’s basically a one stop shop for an iSCSI SAN. The Atto initiator is currently only sold with their iSCSI bridges (iSCSI to FC, or iSCSI to SCSI). This is geared more towards people wanting to repurpose existing FC or SCSI hardware.
As far as FCP implementations, I can’t speak for Atto, but I do know we (SNS) have a 40 seat iSCSI SAN running at the University of Texas. We also have a similar 32 seat iSCSI SAN running in the UK at Teeside University. Both systems are all OS X, all FCP. There are a host of others, but those are just a few.