If you're planning an exposure longer than a few seconds, hot pixels are an issue, and you need to think about dark frame subtraction. Done manually, this means that in addition to your picture, you take an identical exposure with the lenscap on, and then you subtract the dark frame from the picture. This corrects the pixels that are too bright because of electrical leakage.
Most newer DSLRs (but not the original Digital Rebel/300D) will do this for you if you let them. Simply enable long-exposure noise reduction (a menu setting on the camera). Then, after you take your picture, the camera will record a dark frame with the shutter closed, perform the subtraction, and store the corrected image on the memory card. Voila - no hot pixels.
The drawback of this technique is that it takes as long to make the second exposure as the first one, so you can only take pictures half the time.1 That's why many of us prefer to take our dark frames manually.
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