I think whoever suggested about cults following obvious works of fiction had a good idea. That, and the fact that worship of 'old' gods, like Neptune, or what have you, will never really 'die'. It'll just change, go underground, etc. -- you don't really advertise that you're a Chaos worshipper on your resume.
I think back to George Carlin's bit about worshipping the Sun as opposed to any other religion. Why? Because I can see the Sun, I can feel the Sun, it's predictable in what and when it does things.
EDIT: One thing I just thought of is the way a 'cult' - in the true sense of a small group of people following a set of beliefs, not the standard gaming definition - might work in D&D.
A guy starts out having some kind of horrible distress. Out of nowhere he just invokes some other god's name. Some how things work out.
It's like the lucky charm effect. Heck, I had a computer that had constant problems. After I'd explored about every option, I gave my computer a Name. Surprisingly, after that, very few problems.
I could see someone somehow imagining things have True Names of Power, and gathering likeminded people.
Sorry. Was a tangent, this whole post. Maybe there's some insight in there tho.