It ALL comes down to the individual state or states you are licensed. Many states do not license by disciplines, so in those states we are all PEs who are authorized to practice engineering. Bound by a code of engineering ethics, we (PEs) agree that we will only practice in the areas where we are competent. Some states define competency in different ways (i.e. education, testing, experience, etc.). I know Mechanical Engineers who practice Structural Engineering and vice-versa, b/c in the state where they are licensed, and as long as they are confident in their work, they have every right to seal these designs.
In other states, like some of you have mentioned, they do license PEs by discipline. Some of these states include Arizona, Louisiana, Nevada, District of Columbia, and Hawaii. For these states where I am registered, it specifically says on my seals that I am a Registered/Licensed Professional Engineer in Structural Engineering (in some form or fashion). In other states like California, Washington, and Illinois, they specifically recognize a SE. That doesn't mean ONLY a SE can design structures in those states that recognize the SE, but in most cases those states define limits (building size, height, or classification) on what a Civil PE can seal structurally and what projects a SE must seal the docs. It generally comes down the complexity of the project as the limits are defined.
TehMightyPirate...upon successful completion of the NCEES Structural exam, you will be a Structural Engineer, but recognized in your state of Maine as simply a Professional Engineer. It is certainly acceptable to provide your signature as John Doe, PE - Structural Engineer. As this list your licensed credentials (PE in Maine), but also lets those know that you practice structural engineering as opposed to the other disciplines of engineering. Once you pass the NCEES exam, you may be interested in seeking the Model Law Structural Engineer (MLSE) designation from NCEES which will help further define your experience and qualifications. It also helps in some states to speed along the licensing process when seeking reciprocity or comity to become licensed in other states especially if it is a state that licenses by discipline or recognizes the SE designation.
Also like JLNJ stated, most (if not all) Structural PEs who are not licensed in a state like CA, IL, or WA that specifically recognize a SE license, only use PE in the signature credentials, b/c using SE otherwise would lead some to believe you are a SE registered in one of those three states, where in reality you are not. Keep in mind also that the path in most of the SE states to become a licensed SE requires a PE license first in Civil and then you reapply for the SE license after having met additional requirements specific to each state.
And yes, as some have mentioned as well, if you are corresponding (letters, emails, etc.) out of state to another state where you are not licensed as a PE...be careful. You should be clarifying what state(s) you are licensed to practice if sending correspondence to a state in which you are not licensed, as this can also be misleading to others reviewing your work. It can also be taken to mean that you are offering engineering services in the recipient's state which can be a direct violation of many state boards where if caught, you could be subject to disciplinary action and fines.
Again, be sure to pay attention and read your state's laws and any other state's laws in which you seek to be licensed or upcoming projects you may be seeking to provide a proposal for engineering services. In many states, not only do you need an individual PE license in that state, but your firm or company must be licensed to practice with the state's engineering board and authorized to do business with the specific secretary of state... and this includes advertising to provide engineering services or bidding on upcoming projects as well. In other words, in a lot of states, it's illegal for an engineering firm to submit a proposal for engineering services for a project in another state(where the firm isn't licensed) and THEN become licensed once the project is rewarded.
