I agree that you want to be careful mounting an electric motor to a front-wheel suspension. That said, a lot of people have done it - myself included.
I would definitely encourage you to make sure that the front fork dropouts are beefy and made from steel (check with a magnet). Aluminum forks crack and can fail suddenly, steel bends and tends to fail over time.
It's less of a problem with a pedal-first controller (motor starts helping above ~5mph), or one that is a pedal-assist (motor helps when you pedal). Instant-start controllers on larger motors tend to be where the problems are.
On my first bike, I mounted an 800W motor onto a puny steel non-suspension fork on an old mountain bike, and coming out of a stoplight, I gunned it and the motor rolled out of the fork. The wheel twisted, caught in the brake and the bike stopped suddenly and I slammed into the handbars - which left a rather large bruise and was a bit embarrassing and left me stranded several miles from home. Not my finest hour.
My new bike has a roughly 900W motor on a beefy steel fork with an instant start controller. It has a suspension, but that hasn't seemed to cause any problems. I'm sure I have well over 1000 miles on this bike (I ride it 10 miles a day, 3-4 days a week whenever the weather is warm since last summer). It's been fine.
I would highly, highly recommend a torque arm for any motor over about 500W mounted on the front forks. Something like this:
http://www.poweridestore.com/Hub-Motor-Torque-Arm
The rear drop-outs on most bicycles - particularly mountain bikes - are usually much thicker and better reinforced than the front. But even mounted there, I wouldn't recommend an aluminum or alloy frame. Steel is best.