Of all the ways that George W. Bush has been disrespected, calling him by his middle initial doesn't really register.
On the broader point, presidents are politicians, not gods. More to the point, they are our employees, and they should be treated with the dignity and respect appropriate to that position. You don't bow and scrape to your employees, but you don't call them rude names and spit at them either, even if you disagreed with the decision of the hiring committee to hire this particular candidate for the job.
Never confuse the person with the office he or she occupies. Also, as hugh3 pointed out, your president works for you (he was never my president), not the other way around. Do you have the same respect for your mayor? Or your building manager? It's the same job, with a different scope.
