CarolynWright:
Therefore, if your use of a trademark does not infringe or dilute the mark or fails under a noncommercial fair use, then you may use the mark without concern.
Very true. However,
backslash23:
I will not actually be displaying them anywhere except to customers as a sample.
That use may not be noncommercial fair use. The secrecy used doesn't matter. I think the question is "do you intend to profit from the use of the trademark?" Since the topic concerns using the trademark in your portfolio, and I'm fairly confident that you'll be using your portfolio to generate business, then this is commercial use.
Steamscenes is a company that produces, among other products, calendars featuring railroad photographs. The Union Pacific Railroad has given this company legal grief by insisting that they license the logos used on it's locomotives which the company photographs and then sells (in its calendars). Union Pacific has even gone after Steamscenes for using the Union Pacific name in the name of it's product. From a practical standpoint, how would
you differentiate calendars with Union Pacific trains from calendars featuring the Burlington Northern Santa Fe railroad, or CSX Transportation, or Norfolk Southern Railway?
Union Pacific has become notorious for it's licensing tactics, forcing a lot of small businesses out of business, so I won't go into it in any greater detail here. There's a Yahoo group that's very active in discussing the specifics of Union Pacific's licensing tactics and the associated legal aspects and impacts, so I suggest going there if you're interested. The lesson learned is there is no easy answer. If it appears to be an easy answer, it's just begging a higher priced attorney to jump in and prove us wrong.
In my limited experience, companies are very cooperative with granting permission to use trademarks. Usually they have to pay to get their trademark out into the public (advertisement). As long as you aren't stating or implying something that isn't the truth ("I am the official Coca Cola product photographer, responsible for every photograph used in Coca Cola's advertising campaigns"

, you'll probably have no problem getting permission, and in this case, I say it's easier to get permission than forgiveness. The only risk you have in contacting the trademark owners public relations department is hearing the words "no." That's a whole lot better than getting a cease and desist letter from their legal department