TIF with 3 bands to data frame

Hello all,

I am new to rgdal. I downloaded a tif file from 
'http://mrdata.usgs.gov/radiometric/data/NArad_U_geog83.tif' which i 
would like to convert into a regular data frame so that i can plot it 
using ggplot2.

when i tried to read the file using readGDAL i get a warning message

NArad_U_geog83.tif has GDAL driver GTiff
and has 2432 rows and 5868 columns
Warning message:
In readGDAL("NArad_U_geog83.tif") : GeoTransform values not available

What does it mean?

Here s the summary of the file:

Object of class SpatialGridDataFrame
Coordinates:
   min  max
x   0 5868
y   0 2432
Is projected: NA
proj4string : [NA]
Grid attributes:
   cellcentre.offset cellsize cells.dim
x               0.5        1      5868
y               0.5        1      2432
Data attributes:
      band1           band2           band3
  Min.   :  0.0   Min.   :  0.0   Min.   :  0
  1st Qu.:255.0   1st Qu.:255.0   1st Qu.:255
  Median :255.0   Median :255.0   Median :255
  Mean   :222.4   Mean   :231.1   Mean   :226
  3rd Qu.:255.0   3rd Qu.:255.0   3rd Qu.:255
  Max.   :255.0   Max.   :255.0   Max.   :255

Can anyone please tell me which packages i should look into for 
converting it into a data frame so that i can plot it using ggplot2.

Thanks,
Swagath