Jefe, As I recall from an earlier discussion here, having salt in the water does lower the freeze temp, but only negligibly. I realize that you asked for Richard's assesment, complete with the need for some asprin or a strong drink to help the info go down( )

Not fully trusting my memory, I did a search and found this thread http://www.poolforum.com/pf2/showthread.php?t=3355. On page 7 (post #62) MassAttack asks a similar ?, Radish responds (post #68) 'that the difference is negligable' and Poolsean (post #70) says that @ 3000 ppm salt the freeze temp drops from 32 to 30.7 deg (F).

I'm sorry my answer isn't as involved as one that Richard could give, but I'd have to say that @ 3500 ppm the freeze temp will be just under 30.5 deg.

There's the 'quick and dirty' answer, I'll leave it to Richard, or one of the other chemists to give the 'whole story'