I'm not sure why this wasn't part of Sony's presentation yesterday. Maybe the company left it out because it simply matched Microsoft instead of under-cutting it. But, hey, the PS4 has a 500GB hard drive.
UPDATE: Sony's Shuhei Yoshida has confirmed that, like the PS3, the PS4's HDD will be upgradeable.
The tidbit was revealed in a media release sent out by Sony after its press conference had wound down, which also let us know that the PS4 camera would be sold separately for $US59 (surely a big reason why the PS4 was able to be $50 cheaper in Australia than the Xbox One, which includes a Kinect).
You do, however, get a mono headset and HDMI cable with every PS4 sold, the latter being always welcome with a HD system.
You can read the specs sheet yourself here.