I'm pretty much in the same boat as kmonster. Been mtn biking and road cycling for years, but just started with motorcycles. If a car with manual transmission counts though, I've got about 120k miles under my belt in the last 4 years. When I took the MSF class I had no trouble whatsoever, it felt like second nature for all of it. My 2 biggest concerns for this new bike are 1) the weight - its going to be a bunch more than the 250cc from the class (not to mention my 22lb mtn bike!) and 2) everything else around me. Other cars, animals, bad road conditions and especially deer.
Welcome to the site guys! Nice to have some other mtn bikers on the site! I used to BMX when I was a kid, and just got back on two wheels sans motor with a Cannondale for the single tracks here in central TX. Where are you guys located? ccope, you mentioned deer. Leads me to think you are in a mountainous area.
For the weight, I would not worry about it too much. Manufacturers go through a lot to make sure the bikes are balanced. Obviously I have not ridden one just yet, but I imagine it is going to wear it's weight well. All of the mass in down low, between the wheels, not much up top. You will just have to be careful your first couple hundred miles on the thing, but you will get a feel for it pretty quickly. Carrying some skills over from cycling, you will take to it like a fish to water.
The second concern is really the one that is all on you. There are innumerable obstacles and hazards out there for a motorbike rider. No one can make you safe, but you. That is the beauty of motorcycling for me. It is constant attention that you have to pay the bike, and the roads. You cannot let yourself do anything else while you are on the bike. As soon as you become complacent and stop paying attention, someone will merge into your lane without noticing you at all, and it is up to you to be in a situation where you can escape. If you are somewhere else in your mind, and cannot react soon enough, that is on you. The car will have a dented fender, and you will have a dented body. On a bike you have to be alert all the time, and unfortunately, assume that every car is going to merge into you, pull out in front of you, and act accordingly.
I don't mean to be a downer at all, because it is totally worth it! A bike on a beautiful day is just about as good as it gets.