Additional information:

Versions of packages virt-manager depends on:
ii  libatk1.0-0           1.22.0-0ubuntu1    The ATK accessibility toolkit
ii  libc6                 2.7-10ubuntu3      GNU C Library: Shared libraries
ii  libcairo2             1.6.0-0ubuntu1     The Cairo 2D vector graphics libra
ii  libffi4               4.2.3-2ubuntu7     Foreign Function Interface library
ii  libglib2.0-0          2.16.3-1           The GLib library of C routines
ii  libgtk2.0-0           2.12.9-2ubuntu2    The GTK+ graphical user interface
ii  libpango1.0-0         1.20.1-1           Layout and rendering of internatio
ii  librsvg2-common       2.22.2-2           SAX-based renderer library for SVG
ii  python-dbus           0.82.4-1ubuntu1    simple interprocess messaging syst
ii  python-glade2         2.12.1-0ubuntu1    GTK+ bindings: Glade support
ii  python-gnome2         2.22.0-0ubuntu1    Python bindings for the GNOME desk
ii  python-gtk-vnc        0.3.4-ubuntu1~ppa1 A VNC viewer widget for GTK+ (pyth
ii  python-gtk2           2.12.1-0ubuntu1    Python bindings for the GTK+ widge
ii  python-libvirt        0.4.0-2ubuntu8     libvirt python bindings
ii  python-urlgrabber     3.1.0-4            A high-level cross-protocol url-gr
ii  python-virtinst       0.300.2-0ubuntu3   program and library to install vir
ii  python-vte            1:0.16.13-1ubuntu1 Python bindings for the VTE widget
ii  python2.5             2.5.2-2ubuntu3     An interactive high-level object-o

Versions of packages virt-manager recommends:
ii  hal                  0.5.11~rc2-1ubuntu6 Hardware Abstraction Layer

Also, when clicking 'Delete', sometimes get:
libvir: QEMU error : 
libvir: QEMU error : 
libvir: Domain error : invalid domain pointer in virDomainGetID
libvir: Domain error : invalid domain pointer in virDomainGetName
libvir: Domain error : invalid domain pointer in virDomainGetID
Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/console.py", line 393, in retry_login
    self.try_login()
  File "/usr/share/virt-manager/virtManager/console.py", line 435, in try_login
    if self.vm.get_id() < 0:
  File "/usr/share/virt-manager/virtManager/domain.py", line 83, in get_id
    return self.vm.ID()
AttributeError: 'NoneType' object has no attribute 'ID'

But after creating a new machine with the same name as the one just deleted, clicking 'Finish' yields:
libvir: QEMU error : 
libvir: QEMU error : 
libvir: QEMU error : no domain with matching uuid
Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/create.py", line 601, in finish
    self.connection.tick(noStatsUpdate=True)
  File "/usr/share/virt-manager/virtManager/connection.py", line 616, in tick
    vm = vmmDomain(self.config, self, rawvm, uuid)
  File "/usr/share/virt-manager/virtManager/domain.py", line 48, in __init__
    self._update_status()
  File "/usr/share/virt-manager/virtManager/domain.py", line 147, in _update_status
    info = self.vm.info()
  File "/usr/lib/python2.5/site-packages/libvirt.py", line 446, in info
    if ret is None: raise libvirtError ('virDomainGetInfo() failed', dom=self)
libvirt.libvirtError: virDomainGetInfo() failed no domain with matching uuid