Users of Comcast and Verizon are experiencing technical difficulties today in their connectivity to the game. They are experiencing this in both connecting to the actual game, and then maintaining a good connection once in the game.
Blue poster Claudiog is seeking more information from the community on the technical support forums. In particular he's looking for traceroutes from affected users. Head over to their forums (ignoring their own ads) and help them out if you're struck with these issues.
Some users are stating that the Los Angeles datacenters are the culprits for the connectivity issues. However we want to caution users that Blizzard has made no official statement about their datacenters.