While Microsoft and Apple aren't explicit on this issue, Java Swing Look and Feel Guidelines explicitly state that the label/value should be to the right of the control for languages read left to right. The same applies to radio buttons.
The ancient OSF Motif Style also explicitly says that the label is to the right of check boxes and radio buttons (pg9-133). And, while we're doing archaeology, the IBM Common User Access standards states that the label is right of a check box, but they are silent on radio buttons.
Whatever the explicit standards, it’s pretty clear designers are expected to put the label to the right of the control. The rationale is probably what you’re intuiting. When you have a column of check boxes with labels of various lengths, this arrangement allows both the check boxes and the beginning of the labels to be aligned consistently without any big gaps between the labels and the controls, neatly side-stepping the label alignment issues that text boxes and dropdown lists have. In the dead tree world, labels are often to the right of check boxes and radio button equivalents (e.g., bubbles on optically scanned forms), so this doesn’t throw users.
