Ok, well i figured out this cheat as i was getting very bored... Anyway, in the 3rd or 4th mission you have to get new clothes from Binco close by to Grove Street. Well, as soon as you have completed that mission, within 25 seconds, you have to get to Binco, go to the back of the building, and climb up the stairs leading to the roof. Then, a small cut scene will happen. This gives you a time of 20 seconds to find the jetpack, which is located at the bottom-left corner of the roof, behind the ledgey-wall thing.. so good luck and have fun with the jetpack people!!