Some sites are still were not accessible to many on Monday. 
The issue seems to have been resolved as of late Monday.
Leading sites impacted:
United Parcel Service(UPS) www.UPS.com  first recorded 16:00 EDT 9-4-2011
(To contact UPS via Toll Free Number 800 782 7892)
National Geographic http://www.nationalgeographic.com/ first recorded 16:04 EDT 9-4-2011
UK Register http://www.theregister.co.uk/ first recorded 15:58 EDT 9-4-2011

The DNS redirection hack against a company like UPS has the potential to disrupt world commerce.

Other major Internet Web Sites Were Blocked Sunday.

The sites were blocked when hackers infiltrated the DNS records of major websites redirecting them to the hacker's landing page.

People trying to access UPS could not reach the worldwide provider of shipping using www.UPS.com. 

Leading sites impacted:
United Parcel Service(UPS)  www.UPS.com 16:00 EDT 9-4-2011
National Geographic http://www.nationalgeographic.com/  16:04 EDT 9-4-2011
UK Register  http://www.theregister.co.uk/  15:58 EDT  9-4-2011

A hacker group calling itself TurkGuvenligi has declared September 4th, 2011 World Hacker Day.
The extent of the cyber attack has not been determined as reports of other downed sites continue.
Attacks appear to be DNS redirections.