This is good information - thanks!
There's also a Drobo B800i, which is an iSCSI-only device with two Gigabit ports (that can only do iSCSI). The benefit of this box is that it can directly communicate with two discrete iSCSI initiators, which can concurrently access LUNs homed to either Gigabit port. This device would allow the ESXi host to maintain its own datastore on the B800i, while letting the OS X VM use a separate LUN via iSCSI for its own storage, utilizing the other B800i's port, and both the ESXi host and the OS X VM could use the same B800i box concurrently. Incidentally, Drobo includes an iSCSI initiator for OS X with the B800i's dashboard. The downside of this solution is the price, which is $2,500 USD for the diskless device.
So, it appears that the Synology is the way to go for now - it can be had diskless for $530 (with only two bays, though).