japhy,
The shortest distance between two points on a sphere (forget that Earth is squished a bit) is the arc of the great circle passing between those two points so you may have reason for concern. You should read this and this (Cross-track distance). You may find that treating the surface of the sphere as a flat plane acceptable if your talking about a small enough area - otherwise, you need non-planar math.

Regarding your statement about MySQL query - I assume have no idea what built-in trig functions are available and I don't envy re-implementing them from scratch. If this were Pg, you could just use plperl and use the module that did what you wanted :-)