First off, I would love to use PostGIS, but at the same time, the
applications i'm working with already use MySql, and it would be very
difficult to change them.

I am kind of new at this, but MySQL 5 will support the
Distance(geomotry, geometry) function. Now, since MySql doesn't
support doing calculations in different coordinate systems with
different projections, what good is this function when you are dealing
with points on the earth?

I mean, how can i get a real distance if MySql treats everything as
points on a plane?