For the most part, I agree with Lee here.

It is quite possible to trademark a model's name associated with, for example, adult content. I'm not sure what the exact class would be, but there would be a separate class for adult performances on a DVD and adult performances on video on a website, and each would be eligible for trademark registration.

However, it is not possible to trademark anything; one has to, as Rudebox said, show that the name has been used in commerce in the same class as the trademark application, the mark cannot be a common word except under unusual circumstances, and, in the US at least, if another person files an objection to the mark, then the mark can be refused. For example, either saying it's a common-use word that doesn't identify a specific product, or else saying that someone else used the name in commerce in the same class before the first use of the applicant.

So it's quite possible for a model to trademark his or her name if s/he has appeared in a video or otherwise done some work in whatever class the mark is being registered under.

Now... realistically, regardless of who has registered the mark, it is highly unlikely that any but the very largest gay studios would be able to afford to mount a trademark infringement action against someone using the mark due to the high cost, and if the use of the mark is to sell the model's photo sets or videos, there's virtually zero chance that an infringement action would succeed. So for practical purposes, you can probably use a model's trademarked name regardless of whether or not you have permission, though I would always seek it out.

Also, as Rudebox said, anyone can slap a (tm) on something and try to claim a trademark, but doing so is meaningless unless a trademark application has been filed, prosecuted, published for opposition, and finally registered. Only once the mark is actually registered is any protection afforded.