(so you have the original,
unblurred image in the background layer, and two blurred copies of this
layer) Set the layer blending mode to Difference for all three
layers. Now, leaving the top layer copy alone, go to
the first layer (the one that's highlighted in the
image) and fill the area outside the text with white.
Do the same with the original, unblurred copy. Now, go back to the
first layer copy (the one with white) and invert it (Ctrl-I)
Ta-da! You should now see something like the image below.

ADDITION:
You might want to have fun rendering Lighting Effects in the first layer
copy, using the transparency as the texture channel. You'll end up
with slightly more chiseled-looking text, like this:
