Maybe this is normal, maybe not. I just never noticed it until I tried long exposures this month.
I noticed there are probably 20+ (red, purple, white) dots that will show up in the same spot every time I do a long exposure with a dark background. The longer the exposure/higher ISO, the brighter they become. It happens with every lens, so I think it's the sensor. To test it, I decided to place the lens cap on for total darkness, and shoot varying exposures.
1 sec @ 100 ISO no issue.
2.5 sec @ 100 ISO very faint
30 sec @ 100 ISO obvious
30 sec @ 1600 ISO more obvious
Enabling "Long Exposure Noise Reduction" completely eliminated it @ 30 sec/100 ISO.
Here's a link to see what I'm talking about:
http://www.flickr.com/photos/56043702@N ... otostream/