I can tell you that at this time, at least with the two machines that I have tried, Xen and Drobo Pro don't play nicely. I was really bummed.
I have tried 3 different interfaces to the Drobo for iSCSI connection, all with the same results, Xen freaks out beacuse of errors with the data transfer. I am sorry for the lack of detail, I don't have time to get to my Xen test server and try it again. Got the same errors from 2 different motherboards, both AMD CPU, one Gigabyte board, other is ASUS (I think).
Also tried from 3COM NIC, all showed the same error on the /var/log/messages file.
No problem using iSCSI from Windows XP with Gigabyte board, (same nic as one of the Xen servers I have) and from my Macbook Pro. Both yield sustained speeds of >20MB/sec write speed to the Drobo. Next on my list is to try it for Final Cut Pro and see what happens...