So what is the shape of a photon's gravity field? OK, these are massless particles, but they have momentum and energy. I'm guess it's not "spherical" as a photon's energy-momentum tensor is somewhat anisotropic.

Gravity fields don't have a shape, they extend out forever getting less and less powerful as you get further away from the center of mass of the object.

Photons don't produce a gravitational field, they're "affected" by gravity only because spacetime is warped by gravity.

What would happen if you converted a certain amount of mass into a bunch of energetic photons, maybe in a matter/antimatter reaction, or something along those lines. Does the gravity "disappear" then? Example: Convert 1kg into a tightly-concentrated burst of energy. Does that burst generate any kind of warping in spacetime?

Hmm. I've just had a bit of a revelation, and my question is not one that is presently answerable (at least, as it is phrased) as there is no established quantum theory of gravity. GR is a classical theory, and a photon is a quantum construct. You therefore cannot examine one's properties in the context of the other. So, while you cannot ask the question do photons (meaning particles) produce gravity, you can ask the question does light (meaning wave-like light) produce gravity. And the answer to this latter question, is yes. So, to take Jeff7's question; if you convert 1kg of matter into a burst of energy that is emitted in a symmetrical spherical pattern, then at the instant of conversion the energy has the same gravitational effect as the original mass did. Perhaps I could rephrase my original question, in the light of the above as: what happens next (as the energy starts to travel outward)?

pun intended? Why wouldn't it be conceptually the same as what happens when a 1 Kg ball of mass is exploded outward in a symmetrical spherical pattern (subtracting out the energy of the explosion)?

How do you know? I don't remember that from the one quantum class I took, so I'd like to read more if you have any information.

This is a GR concept, rather than a quantum concept - and as the two are mutually incompatible.... Anyway, I believe that this is a result of Birkhoff's theorem. Although, I'm not able to even begin to solve the EFE for myself.

Some quantum electrodynamics may help answer - or at least provide further reading Mark... http://en.wikipedia.org/wiki/Photon#Contributions_to_the_mass_of_a_system

Back to the OP - in whose frame of reference? The photon's frame of reference? Or our frame of reference?