First, let me provide perspective by saying that I have only been backcountry skiing for about 4 or 5 years and that I only get in at most a dozen or so touring days per season.
I guess I think that the requirements to "try" backcountry skiing that have been mentioned here are perhaps a bit overstated. In my experience it was very easy to try it without a huge investment. My family and I hired a guide service and skied of Teton Pass. All AT/randonee gear was rented. Two of us rented skies (w/skins) with AT bindings and two of us used our own skis (w/ rented skins) with rented Alpine Trekkers (AT binding adapters). We all had packs that we had previously bought that included a method for carrying skiis (2 Dakine Heli-Pros's, 1 Wookey Pack, and 1 other). We had these packs from previous need based on guided and unguided hiking from inbounds. We also used beacons shovels and probes that were provided by the guide.
It was very easy to pick out appropriate clothing layers from our existing ski clothing although we have certainly over time purchased more efficient and effective clothing items better suited to back country. However, there is almost no piece of clothing that I haven't used for both backcountry and inbounds skiing with many new "backcountry" items working better inbounds than anything I had previously owned.
In terms of avalanche knowledge I do not think it is at all necessary to take an AVY I course before you have some experience - that is - as long as you go with a qualified guide who is well aware of your level of knowledge. Certainly some intorductory information and beacon training should be provided by the guide. Most important, however, I think is that you should have some awareness and interest about avalanches, snow pack, and route finding. www.avalanche.org
is a great resource with a good library of online materials for you to learn from before you head out on your first trip. Also, Bruce Tremper's book Staying Alive in Avalanche Terrain
is very good.
I have been extremely lucky to have made a number of (very experienced and expert backcountry skier) friends who have been willing to take me touring. Through them, a couple of formal guiding experiences, a steep skiing camp, and reading I learned most of the stuff provided in an AVY I course. I and my two kids finally took an AVY I course this January and while it was review for a lot of things it was very effective in helping me consolidate my previous experiences. While I expect to take an AVY II course in the future, I don't think it would be a bad idea at all to repeat the AVY I course again (which a few people in the course were doing). The thing is that I believe the most important aspect of safe backcountry travel is route finding based on a good understanding of snowpack history and weather. You really need to be doing a lot of touring to get this kind of experience. At this point I almost always go backcountry with VERY
experienced friends. I almost never go on my own or with my kids (who essentially have the same level of experience as I do) unless it's an area that we are familiar with and about which we have a reasonable understanding of weather and snowpack. Perhaps that is just starting to change but as of now it is still true.
In my experience, if you are outgoing and can demonstrate some basic knowledge and understanding, it is pretty amazing how you can find people willing to take you out on a tour. Obviously you need to be able to judge as to their level of expertise but that is not all that hard. Before that, there are numerous opportunities to hire guides and rent touring equipment. After a couple of trips, if you are so inclined, you will probably want to get a pack, beacon, shovel, and probe of your own.
If you would like some suggestions about places and guides for some backcountry touring drop me a PM and I will give you the few leads that I am aware of. some of them are pretty pricey but others can be pretty reasonable if you have a group to split the costs.