If off. of def. players line up in the netural zone, is it a dead ball or live ball infraction?

In NCAA, an offensive encroachment is a deadball foul and a defensive offsides is a liveball foul

In Nat Fed football (High Schools in all but two states I believe), all encroachment is a dead-ball foul.

More on encroachment
 

Under National Federation rules, encroachment is always a dead ball foul.

Encroachment restrictions begin after the ready for play signal for:

1. The snapper if he lines up with one or both feet in the neutral zone (watch for this in the swinging gate formation on the try). Rule 7-1-1.
2. The snapper if any part of his body (except for one or two hands in contact with the ball) is beyond the ball on his opponent's side of the neutral zone. Rule 7-1-1.
3. Other players if they touch the ball. Rule 7-1-5.
4. Other players if they touch an opponent. Rule 7-1-5.
5. Defensive players cannot give defensive signals while in the neutral zone (umpires can guard the ball to prevent this). Rule 7-1-5.

Encroachment restrictions that begin after the ready for play and the snapper has placed his hands on the ball begin for everyone if:

1. Any offensive or defensive player breaks the plane of the neutral zone (whether touching the ground or not). Rule 7-1-6.