Kurt, you can group the temperature values in your raster to classes and export the results to a new raster using v.reclass from the Sextante toolbox.
I guess the minimum value of your interpolated raster might be (say) -5 and the maximum value (say) 30.
Using GRASS v.reclass from the Sextante toolbox would allow the values to be grouped into seven classes using this 'rules' text file (you could call it 'rules.txt'):
-5 thru 0 = 1
1 thru 5 = 2
6 thru 10 = 3
11 thru 15 = 4
16 thru 20 = 5
21 thru 25 = 6
26 thru 30 = 7
The output would be a new raster having a value of 1 for all of the values of between -5 and zero in the original raster, of 2 for all of the values between 1 and 5 in the original raster, and so on.
The procedure is very simple, all you need is the interpolated raster and the 'rules' text file. See also the Man page for v.reclass here: http://grass.fbk.eu/gdp/html_grass64/r.reclass.html
Once classified, the new raster could also be polygonised to produce a polygon shapefile, to put hard edges on the colour-rendered image. Or you could colour style the shapefile and forget about the raster.
Just a quick note. Interpolation is one of those things that makes what's left of my hair stand on end because it can produce very convincing-looking results from very thin data. What's more the results are usually impossible to check because you've used all of the data you have to do the interpolation, so it's in the nature of things that you can't carry out meaningful checks on the areas for which you don't have data.
In your case, the data for the area outside of the borders of Austria is thin and you might consider clipping the final map image to show just Austria. Or maybe leave the points in. For example, I might have a graph with a shotgun splatter of points through which I draw a straight line. The dishonesty starts when I then remove the points :)