Yes...you are supposed to state your LLC or Incorporated status on all advertisements and other public documents such as checks, bills, etc. (Make sure you use the right one: either "LLC" for a Limited Liablity Company, or "Inc" for a corporation.) The reason for this is that it puts the public on notice that the individuals with which they are dealing have limited liability as to the debts (and some of the liability of) the company. If you don't do this, then a court could rule that you have "pierced the corporate veil", and that you are personally liable for the debts and obligations of the company. In other words, the court could rule that the public was lead to reasonably believe that you and the company are indistinguishable (i.e. a sole proprietorship), and therefore they will treat you as such. You definitely don't want that to happen.
P.S. This even applies to truck/van markings. Leaving it off of one or two things isn't going to hurt you in and of itself, but if you leave it off of too many things it could. Best to play it safe and list your limited liability status on everything. Better safe than sorry.
[This message has been edited by TNSunny (edited 06-30-2006).]