A 257: Pretty sure that's a no. The thunderlance isn't technically a weapon, thus not a target for magic weapon. Even if it were, I wouldn't consider that a very efficient use of your spells. Magic weapon is typically cast as a buff before battle since it has a longer duration. This doesn't mesh very well with Thunderlance which is only 1 round/level. Once the thunderlance is out you should be swinging, not buffing.