Hi Jorrie and Bjarne,
1. It sounds right to me that a comoving observer could measure tidal effects.
2. Bjarne, I didn't formulate my question clearly enough. I should have asked, "Does an electromagnetic field cause geometric curvature of space with respect to a charged particle moving through the field?" In that respect, I note the following from the Wikipedia article on "Maxwell's equations in curved space:"
"In physics, Maxwell's equations in curved spacetime govern the dynamics of the electromagnetic field in curved spacetime (or more generally, spacetime with a non-Euclidean metric). They can be viewed as a generalisation of the vacuum Maxwell's equations as they are normally formulated in the local coordinates of flat spacetime, but general relativity dictates that the presence of electromagnetic fields themselves induce curvature in spacetime, so Maxwell's equations in flat spacetime should be viewed as a convenient approximation."
It sure is interesting to contemplate the idea that an electromagnetic field actually curves spacetime, but in a way that is "apparent" only to charged particles. If it's true, then it reinforces the question of how "real" the curvature of space by gravity is.