Astronomers drove digital imaging sensitivity, so I reckon they’ve got it worked out. Skimming http://arxiv.org/pdf/0709.4281v1 , it looks like Ralph, the main visible-light camera, can distinguish between true blackness and a mere 3k photons per pixel, and at the margin can detect objects of magnitude 14. Coincidentally, that’s about the magnitude of Pluto from earth, and you can’t see it even with a small backyard telescope. So this instrument is less like a consumer camera and more like, say, night-vision goggles.

This paragraph originally did the math for Pluto’s brightness wrong (thanks to atakan_gurkan below for pointing that out), so instead let me refer you to a short but interesting Metafilter thread on exactly this question: http://ask.metafilter.com/23197/Darkness-at-the-edge-of-town .

One more factor – Pluto’s albedo is quite high. It reflects 0.5 to 0.66 of the light falling on it, as opposed to e.g. 0.14 for the moon (which is fairly dark in the scheme of things, but not at all hard to photograph; you can easily overexpose it in a grainless photo with a recent consumer DSLR).

So: dim, yes, but still clearly visible.