Although the GUI development of Wireshark is platform independent, the Wireshark development team tries to follow the GNOME Human Interface Guidelines (HIG) where appropriate. This is the case, because both GNOME and Wireshark are based on the GTK+ toolkit and the GNOME HIG is excellently written and easy to understand.
For further reference, see the following documents:
GNOME Human Interface Guidelines at: http://library.gnome.org/devel/hig-book/stable/
KDE user interface related documents at: http://developer.kde.org/documentation/standards/kde/style/basics/index.html
Win32 styleguides available at: http://msdn.microsoft.com/en-us/library/aa511258.aspx