This is with virt-manager-0.2.3-3.el5

This is very odd, I've seen this lack of decorations every now & then too - the
glade file is clearly showing it as a regular dialog:

<widget class="GtkDialog" id="vmm-preferences">
  <property name="visible">True</property>
  <property name="title" translatable="yes">Virtual Machine Manager
Preferences</property>
  <property name="type">GTK_WINDOW_TOPLEVEL</property>
  <property name="window_position">GTK_WIN_POS_NONE</property>
  <property name="modal">False</property>
  <property name="resizable">True</property>
  <property name="destroy_with_parent">False</property>
  <property name="decorated">True</property>
  <property name="skip_taskbar_hint">False</property>
  <property name="skip_pager_hint">False</property>
  <property name="type_hint">GDK_WINDOW_TYPE_HINT_DIALOG</property>
  <property name="gravity">GDK_GRAVITY_NORTH_WEST</property>
  <property name="focus_on_map">True</property>


Testing again now, about 1-in-3 times, when initially displaying the window
after launching the application it doesn't get any decorations. Closing &
re-openning the window, the decorations are fine.

This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux major release.  Product Management has requested further
review of this request by Red Hat Engineering, for potential inclusion in a Red
Hat Enterprise Linux Major release.  This request is not yet committed for
inclusion.

This is fixed in virt-manager-0.4.0, and should therefore wind up fixed in RHEL
5.1 as well.

fixed in 0.4.0-1.el5, to be precise...

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHEA-2007-0637.html