For art, and I'm pretty liberal on what I'm willing to consider art, sure, without hesitation. I'm a 26 year old man from the northeastern US. I'd prefer my face at least be positioned in a way that it's not readily identifiable (or, if we're talking about drawings or paintings, not too realistically rendered), but that would be my only request. In fact, I would not only agree to do it, but I have done it, first in the SDMB thread (inserting myself into male nude classical art), and after having such a fun time with that, I started a blog along the same lines and did it a few dozen more times. I'll link to it if there's any interest.
As for unabashed pornography, that's a very-firm-(yes-ha-ha)-but-not-absolutely-set-in-stone no.
Tangentially related:
Is everyone here aware of the
Human Variation Project? The owner seems to have abandoned it, but it was very interesting while it was operational. It was a blog that invited ordinary un-model-like people to submit nude photos of themselves.
There's also
ErectionPhotos.com. Same idea, but with the focus narrowed on penises, particularly on their various angles and curvatures.
I'm not on either of them, before anyone asks.