[Thanks to everyone who sent this in]
Update: Dish has a new webpage up to help customers deal with the situation here, while those who can't get through on crowded helplines are advised to check out @dish_answers on Twitter.
Update 2: Dish's site linked above indicates it has "identified the issue" and estimates it will be fixed overnight.
Update 3 (9/14): According to Dish Network's official Twitter account, things should be up and running again now. Satisfied? P.S. - Another helpful DBSTalk post recommends a guide data update by pressing Menu, then 999 and then the right arrow.
ENGLEWOOD, Colo., Sept. 13, 2011 - We are currently experiencing an interruption affecting the 129 orbital satellite location causing an HD signal outage in some areas. We have identified the issue and based on our current estimates, the situation should be corrected overnight and your HD channels should be restored in the morning. In the meantime, you can unplug your receiver from the main power source and plug it back in to see your standard definition channels. We apologize for this inconvenience.