Do any of these extra paned windows provide features that don't exist in the native Tk panedwindow?
EKB Not exactly, but... I decided not to migrate away from the iwidgets paned window (yet) because the Tk 8.4 version doesn't have the iwidgets "fraction" option. Instead, you have to specify the size of each pane. Setting "fraction" instead is quite handy. Of course, this isn't a show stopper, but it made the switch nontrivial.
EKB: Oh, right... And more importantly, there is no hide command for the Tk 8.4 paned window. Again, it's not a show stopper, since the behavior can be duplicated, but it slows me down when converting scripts.
LV: Might be worthwhile to add these two missing features to the Tk Features Request queue over on sf.net .
[Does Perl/Tk have a Pane widget?] VK: Perl/Tk has Tk::Adjuster, which, according to docs, Allow size of packed widgets to be adjusted by user. Strangely, it has Tix mixed in, but PanedWin from Tix was not used, for some reason it has its own... Some kind of implementation details.