SteveWh - 4:49 am on Dec 21, 2010 (gmt 0)
Once a site gets overrun with parasite hosting spam beyond a certain point, Google already does delist the site for 30 days, and sends the webmaster an email explaining why. They've been doing that for a long time.
So unless something has changed, these new warnings must be in addition to that. Maybe they're what happens before the situation gets so severe that they delist it.
When they delist a site for 30 days, the site gets back into the SERPS after the 30 days has passed, if the problem has been resolved.