sanboot iscsi fails. Any idea why?

[ingestre]

I've been knocking my head against a sanboot iscsi issue which I'm struggling to resolve. Perhaps one of you fine technical people could shed some light?

I have a FreeNAS server which is presenting an ISCSI target quite happily. I can attach this as a second drive to my existing machines. What I'm trying to do now is get a machine to boot off it.

The source installation is a Windows XP box with Microsoft ISCSI initiator installed, as well as the sanbootconf.msi iscsi boot support

I've used Clonezilla to image my source installation onto the iscsi target (disk to disk imaging, including the MBR)

I then boot into the iPXE environment (I use pxelinux to boot the iPXE ISO image, hitting CTRL-B to get the shell)
I run "dhcp" to get the network interface configured
I then run the following sanboot command

sanboot iscsi:192.168.0.5::::iqn.2011-03.org.example.istgt:thinclient

I get a couple of messages saying that the iscsi target has attached as drive 0x80 and that iPXE is trying to boot off it. At this point the machine appears to lock up, and a reboot is the only option.

Has anybody got any idea what the problem is? I fully accept that I may have done something dumb here - I'm just getting started with iPXE / iSCSI and have a lot to learn!