The issue is that when you do a concentrated wort boil, you inhibit the extraction of the bitterness from the hops. When you boil in pure water, you're getting as much bitterness from them as possible (and the more water you boil in, the higher the extraction).
The other, I think better, way of approaching this is to add the majority of your extract to the boil in the last ten minutes. Extract doesn't need to be boiled 60 minutes, only 10 or so to ensure sanitation. Doing this allows the hops to have their alpha acids extracted to a greater degree, since they're being boiled in a lower-gravity wort for most of that time.
As a side benefit, the beer will be a lot lighter.
The danger, as has been mentioned, is that it is easy to overdo the bitterness if you're not careful.