I'm having this problem also. While the demo for Ratchet and Clank was installing, I switched video inputs on my TV to watch something else during that time. When I tried to switch back, I get no HDMI output from the PS3. I can get output through component using the auto setup, but if I try to use it again for HDMI there still is no picture output.
EDIT: Aha, figured it out! When doing the auto setup with both component and HDMI plugged in, on the component input, the PS3 showed the screen where it said it detected an HDMI cable and asked me if I wanted to use that for audio and video. After I saw that, I knew that it must be a fault of the TV, and not the PS3, as it is detecting the HDMI. So I unplugged my TV, let it sit for a minute, and plugged it back in. Then, I went to the HDMI input on the TV for the PS3 and once again started the auto setup for the PS3, and this time it worked. So if you haven't tried it yet OP, try unplugging your TV for a minute and plugging it back in.