Quick question - did you shoot in RAW and convert to JPEG for posting here? Or did you shoot in JPEG mode? Keep in mind that a sensor is like a red/green/blue checkerboard that has to "guess" color differences - especially darker ones where it's only a slightly different hue vs a whole other color altogether. Add to that a JPEG compression (they've gotten much more efficient, but let's face it, it's compressed) and you have strange "noise" that should be a clean surface.

I have a friend with a 20D, and when she shoots in JPEG mode she gets quite a bit of noise in darker areas. When she shoots at ISO400 (which is too often!) it gets pretty noisy in general!

I can tell you that in print ads that would be hidden by the printing pattern (grab ANY publication - especially lower-cost ones, and newsprint - and hold it REALLY close!) so it's not a concern (which is why many magazine articles almost dismiss the noise at those levels until you get up to ISO 1600 where it's significantly "chunky" and won't hide in the printing process), but for inket prints and light-jet type printing it would show up.

If you shot in JPEG mode, try shooting in RAW and convert to TIF (or Bitmap - either is uncompressed) - then look at it in Photoshop (or like app) at 100 percent and see if it's still there.

Good luck-
-Lew ;-)