Photofocus Question - Tilted Horizon

Oh yeah... One of my questions was 'answered' in the Photofocus podcast #09 by Scott Bourne and Rick Sammon (last question of the episode).

My problem' is that when I shoot in portrait mode that the horizon is always a couple of degrees off. The right side of the image is significantly lower than the left side. No matter how hard I try. Even though the problem is easily corrected in post-processing, it's annoying as hell.

Scott suggested a using the virtual horizon (which isn't present on the D300.... hmmm which gives me an excuse to upgrade ;-)), or using a hot shoe bubble (which is a whole lot cheaper). Rick mentioned possible lens related distortion. It's not the lens, because the problem exists in the range of 10mm wide-angle to a 200mm telephoto. So I'll be trying a hot shoe bubble for a while. Let's see if that helps.