Anything that gets your heart rate up and gets you moving, will help with weightloss.
When my husband was in college, his parents still had their trampoline in their back yard, and when he'd go home during the summer, I spent a lot of time there, and a lot of time jumping on that trampoline everyday after work....and did no other excercise (Sp? I always screw that one up), and I was really fit.....so, I'd say, yes, trampolines are a fun way to lose weight

When we buy a house, I plan to put a trampoline in the back yard, cause I love them, and I'm 29....I don't even care