Likely it's due to brake dust. Do the brakes perform fine? Do you have any idea what kind of pads were used? Have you verified the pads on the offending wheel are in fact replaced (i.e. nice and thick)?
Next time you're on the highway, brake very aggressively when you're slowing down to take your exit (after checking nobody is behind you). See if that clears up the squeal for a short time. If it does, it's likely because you've temporarily cleaned out the brake dust through the hard brakes. Many cars that primarily get driven in cities end up with brake squeal because they rarely undergo high speed hard braking like highway stops. Do you do mostly city driving?