For those of you interested in purchasing an Ebike, or making a bike an Ebike try Lunacycle.com, they have the best prices.
I've done a fair amount of research on the what's legal thing. The Feds consider an Ebike limited to 750 watts (States can set a higher limit, but not lower that the established Fed limit of 750 watts. Ca sets it at 1000 watts) that has a pedal assist feature a bike, and as such can be taken anywhere a non Ebike can be taken. Thus if mtnbiker208 had such a bike, behind that closed gate, it would have been legal, just as a non-electric bike. The key here is that the Ebike MUST have a pedal assist feature (it can also have a throttle feature).
750 watts simply isn't enough to get you up those steep hills. So for the purpose of hunting LunaCycle came up with this beast
Luna 5000w Rhino Cargo - Luna Cycle I believe the standard stock set-up is 3000 watts, not the advertised 5000. Notice that they also use a Mid-drive motor, rather than a hub motor, for good reason.
For those thinking of converting a mountain bike, you will want to run the widest tires you can get away with, simply because you will find that you will need the extra stopping ability of wider tires, normal tires tend to skid and you slide much further than you'll want to. These powerful (3000 watt) mid-drives will push a mountain bike 40 MPH on flat ground. You don't really need the speed, but you do need the power and torque, especially if your carrying a load. IMNSHO, the way to go is a fatbike with a front suspension, but then I'm pushing 54, and want a little comfort.