More on SpamRichi has another fine post on SPAM entitled "Fewer spammers forging the From header". I agree but would add one more thing to this bullet item:
Increasing use of sender authorization technologies such as SPF, Sender ID, and DKIM by spam filters -- spammers think that a valid return address makes it more likely that their spam will get delivered
These technologies, as well as brute force techniques such as Sender Verify, not only reduce the chances of a spam with a fake From address getting through, they cause bounces. Many of the large ISPs are now tracking these bounces and therefore tracking down people using their networks to spam. In a few cases they are even coordinating efforts with competitors. This makes it much more "dangerous" to spam or create a spam bot which uses a fake address.
As Richi rightly points out most abuse desks are greatly overworked so they concentrate their efforts on the most egregious, but all of these slimeballs are getting their software from the same places and sharing techniques to some degree.
Just some more inside baseball.