It should be working in 6.6. I redid many of the ETO based dialogs last week and fixed several things MultiListBox was one of them.
I just did a quick test on a debug build and it appears to work via rhino script and rhinocommon.
import rhinoscriptsyntax as rs
import Rhino as r
a = [ ‘one’, ‘two’, ‘three’ ]
rscriptresult = rs.MultiListBox(a,“Script Message”,“Script Title”,“one”)
rcommonresult = r.UI.Dialogs.ShowMultiListBox(“UI Dialog Title”,“UI Dilaog Message”,a)