I am using Renoise on Kubuntu 11.04 with the KDE desktop, and it seems that for a long time now there have been no tooltips at all in Renoise, even with “Show tooltips” enabled in the preferences. I am sure that there were once tooltips, because otherwise there was no easy way to find out what some of the icon buttons do. There just don’t seem to be any now for some reason.
I have checked on two different machines so it is not machine-specific, but they are both using KDE with KWin window manager, so I suppose it could be desktop or window manager-related.