Does red light preserve your night vision?
This is a very good question.  Red light is routinely used by scientific laboratories to do low light dissections of retinas, and of course it is used in other low light contexts such as printing plate development.

In both of the above contexts, you have a clear subject: the retina being dissected or the printing plate being developed.  In the case of the printing plate the film has been designed to be specifically non-reactive to red light, so red light is used because your eyes can see it, but the film doesn't react to it.  Similarly in some scientific settings it makes sense to use red light during dissections.  Mice lack a long wavelength opsin, and therefore using a dim red light allows the experimenter to have a relative sight advantage compared to the mouse when keeping the mouse dark adapted.

But in the case you're asking about, there is no film or animal to serve as a second party.  So is there any intrinsic advantage to using red light?  As it turns out, there is.  The fovea, which is in the center of our eye and used for high acuity vision, has no rods and primarily L- or red sensitive cones.  Note the high density center area which lacks blue sensitive cones and has 2:1 red to green cones.  



So by having red light present, you stimulate this area.  But red light is present in white light, too, why not just use that?  Leonardo's answer comes the closest, but it's a little off.  Red light is used because it preferentially stimulates L cones more than rods, but you are definitely not able to preserve night vision by using red light.  Why not?  Well it may look like it is possible to exclusively stimulate cones from the chromatic sensitivity figure 



But that figure is 1) normalized and 2) not indicative of synaptic signal processing.  1000's of rods can converge onto a single ganglion cell, where cone convergence in the fovea can be on the order of a single cone per ganglion cell.  When it comes to perception, in order to compare the black rod line above with the red L-cone line you'd have to magnify it dramatically in size.  Practically speaking, it is nearly impossible to stimulate cone pathways without stimulating rod pathways when using a relatively broad spectrum LED that you're powering with a battery.  Maybe with a high power infrared laser.  

So the purpose of using red light is to attempt to balance the activation of high sensitivity (red insensitive) rods with that of the low sensitivity (but red sensitive) cones in the fovea.  While using a similar level of rod activation with blue light, you would perceive a "blind spot" where your fovea is.

Finally, instrinsically photosensitive cells (the melanopsin cells brought up) do not factor into this processing.  These cells are activated only with extraordinarily bright levels of light, and the therefore do not enter into conversations dealing with night vision.