I answered you on the list. Quick version: when you turn on "has-tip", gtk+ will create the tooltip and ask you to set it up and whether to show it in the "query-tooltip" signal.
Comment on Re: Gtk2 Treeview/Simplelist and set_tooltip_row()
Muppet, I was just going to link to that! Thanks again for your help. I think your code should also be here natively in a readmore code tag. Links have a way of aging out after a while.