Description of problem:
Using virt-manager, I added a gfx head to a VM that didn't have one previously.  Looking at the resulting xml, it also changed some of the serial config.  This seems harmless but unexpected.

--- 1	2009-07-06 16:20:40.000000000 -0400
+++ 2	2009-07-06 16:20:43.000000000 -0400
@@ -36,13 +36,15 @@
       <model type='virtio'/>
     </interface>
     <serial type='pty'>
-      <source path='/dev/pts/2'/>
+      <source path='/dev/pts/3'/>
       <target port='0'/>
     </serial>
-    <console type='pty' tty='/dev/pts/2'>
-      <source path='/dev/pts/2'/>
+    <console type='pty' tty='/dev/pts/3'>
+      <source path='/dev/pts/3'/>
       <target port='0'/>
     </console>
+    <input type='mouse' bus='ps2'/>
+    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us'/>
   </devices>
 </domain>

then I removed the gfx head and it also removed a snippet from the network section.  This might be harmless but is even more unexpected than the first change.

--- 2   2009-07-06 16:20:43.000000000 -0400
+++ 3   2009-07-06 16:20:52.000000000 -0400
@@ -32,7 +32,6 @@
     <interface type='bridge'>
       <mac address='00:01:01:11:06:00'/>
       <source bridge='br0'/>
-      <target dev='vnet0'/>
       <model type='virtio'/>
     </interface>
     <serial type='pty'>
@@ -43,8 +42,6 @@
       <source path='/dev/pts/3'/>
       <target port='0'/>
     </console>
-    <input type='mouse' bus='ps2'/>
-    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us'/>
   </devices>
 </domain>

Version-Release number of selected component (if applicable):
RHEL 5.4 beta
python-virtinst-0.400.3-4.el5.noarch
libvirt-python-0.6.3-11.el5.x86_64
libvirt-0.6.3-11.el5.x86_64
libvirt-0.6.3-11.el5.i386
virt-viewer-0.0.2-3.el5.x86_64
virt-manager-0.6.1-4.el5.x86_64