R help - Jun 2012 - ggplot2, grid graphics, x11(), windows(), and device fonts

I am trying to use ggplot() to produce a graph and am getting warnings that 
I don't understand.  This is happening from within RStudio, but also happens
if I start Windows R GUI.   Can anyone help me understand the warning and 
how to make sure the right fonts are designated?  The relevant code fragment 
is:

x11(width=7, height = 7)
library(ggplot2)
# Now some Distance ~ Time plots
p2 <- ggplot(lu.mig, aes(x=Time, y=Net.Distance, colour = PlateRow)) +
  geom_point(size= 3) +
  geom_line(size=1) +
  facet_grid(Substrate ~ Expression) +
  xlab("Time After Scratch (h)") +
  ylab("Net half-distance (um)") +
  opts(title='Repaired Raw 1205Lu Scratch Time Course') +
  theme_bw(base_size = 18, base_family = "")
print(p2)

The warnings are:
There were 22 warnings (use warnings() to see them)
>
> warnings()
Warning messages:
1: Removed 1 rows containing missing values (geom_point).
2: Removed 1 rows containing missing values (geom_path).
3: In grid.Call(L_textBounds, as.graphicsAnnot(x$label),  ... :
  Font family not found in Windows font database
4: In grid.Call(L_textBounds, as.graphicsAnnot(x$label),  ... :
  Font family not found in Windows font database
...  snip ...   # repeat same warning
22: In grid.Call.graphics(L_text, as.graphicsAnnot(x$label),  ... :
  Font family not found in Windows font database

If I comment out,
    x11(width=7, height = 7)
so that the plot is drawn to the RStudio graphics device (or the default 
device in R GUI), I only get the expected first two warnings.  As I have 
traced through the documentation I've seen reference to an 'Rdevga? that
may
be a culprit, but I can't quite understand how I set this up properly.  Any 
debugging pointers?

It also seems that if I replace x11() with windows(), the warnings 
disappear.  Somewhere (wrongly, I guess) I had gotten the impression that 
x11() on the windows platform behaved just like windows().  For now, I've 
just reverted to using windows(), but I'd like to understand this better, 
particularly how to be certain ggplot() (or grid()) will be able to find a 
default font.  Of course, despite the warnings the graph draws fine so it 
could be something to evaluation in ggplot.

Thanks,
Rob
> R.Version()
$platform
[1] "i386-pc-mingw32"

$arch
[1] "i386"

$os
[1] "mingw32"

$system
[1] "i386, mingw32"

$status
[1] ""

$major
[1] "2"

$minor
[1] "15.0"

$year
[1] "2012"

$month
[1] "03"

$day
[1] "30"

$`svn rev`
[1] "58871"

$language
[1] "R"

$version.string
[1] "R version 2.15.0 (2012-03-30)"

$nickname
[1] ""


C:\Users\rbaer>path
PATH=C:\Perl\site\bin;C:\Perl\bin;C:\Program Files\GTK\bin;C:\Program 
Files\ImageMagick-6.7.6-Q16;C:\Program Files\Mende
ley Desktop\wordPlugin;C:\Program Files\Common Files\Microsoft 
Shared\Windows Live;c:\Rtools\bin;c:\Rtools\perl\bin;c:\R
tools\MinGW\bin;c:\Rtools\MinGW64\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\Window
sPowerShell\v1.0\;C:\Program Files\Microsoft Application Virtualization 
Client;C:\Program Files\WIDCOMM\Bluetooth Softwa
re\;C:\Program Files\Windows Live\Shared;C:\Program Files\MiKTeX 
2.9\miktex\bin\;C:\Program Files\QuickTime\QTSystem\;C:
\GTK\bin;


------------------------------------------
Robert W. Baer, Ph.D.
Professor of Physiology
Kirksville College of Osteopathic Medicine
A. T. Still University of Health Sciences
800 W. Jefferson St.
Kirksville, MO 63501
660-626-2322
FAX 660-626-2965