I've wanted a SCSI "disk" that is really an iSCSI initiator that reexports an iSCSI target over a local SCSI bus. Basically a SCSI to ethernet/wifi to fileserver. It would be one solution to the diskless booting problem. The inverse is commonly available for people that have SCSI disk arrays and want to export them as an iSCSI target. But I've never seen the direction I'm looking for.
Here's a tidbits article on TDM:
http://tidbits.com/static/html/TidBITS-094.html
From what I understand, they were ROM replacements for existing ethernet cards that would retrieve an image at boot, copy it into RAM, and present it as a bootable RAM disk. This seems to indicate the TDM solution could use AppleShare and something "unix compatible" (presumably tftp or nfs) for the image retrieval. Apparently some schools used it for easier administration.