A bit more research and the current solution seems to be to talk to the Walkman (NWZ-S638F) via MTP rather than USB Mass Storage. To make this happen you need an updated libmtp for it to recognise the Walkman. Since Jaunty is still at 0.30 version of libmtp I’ve backported the debian experimental version which is the latest v0.34.
This means the Walkman now is recognised and works fine in Rhythmbox - Banshee seems to crash a lot but I’m not sure if this is because of my backported libmtp or because of other mtp bugs in Banshee.
So if you want to use a recent Walkman with Interpid Ibex install the libmtp files linked below.