Paul Ramsey on GEOS 3.1 and PostGIS 1.4 improvements:
Here's a less contrived result, the 3141 counties in the United States. Using the old ST_Union(), the union takes 42 seconds. Using the new ST_Union() (coming in PostGIS 1.4.0) the union takes 3.7 seconds.
Now that's change we can believe in.