Re: When we say F = qv x B what is the v measured relative to?

Your question is a good one, and is one I've never thought of before...

I'm pretty sure that the magnetic force equation was developed based on 
measurements made on the earth, which for practical purposes amounted to an 
inertial reference frame for the cases being studied.  I believe that it should 
also hold true for any inertial reference frame -- one in uniform motion, not 
under acceleration -- as well, and that differences in what would be perceived 
by different observers would end up washing out as the values of the 
electromagnetic fields are re-evaluated in different frames.  Note that I've 
made the distinction between electromagnetic fields and the B-field because, in 
general, different frames may see distinctly different E- and B-fields.