It looks like one of DirecTV's astronauts had more trouble flipping switches than their DISH counterparts, after a software or guide data update last night knocked out HD DVRs all over -- and not for the first time. Currently the only advice needed to get back in working order is to reset the box twice (there's a red button on the front, underneath the flap) within a thirty minute period (the official website indicates you can wait as little as 15 seconds between resets) and all should be well, but good luck getting through to customer service to get that message right now. Countdown to an apology starts... now.
[Thanks to everyone who sent this in]
Update: According to DirecTV's website, a "transmission glitch" was to blame and it has automatically reset affected receivers from its end, so no further red button pushing should be necessary. [Thanks, Blake!]