From the crew end, yes, these message boards work.
My boyfriend and I sailed to French Polynesia from California with a captain we met on a forum like this, and in September, we meet our new captain (whom we also met online) in Bali and sail to South Africa.
When using forums to find crew/boats, I try to be honest about my experience and my desires in a captain, and stay professional in tone. We already had some sailing experience prior to hitching rides around the world with captains we met on the internet, which certainly helped our postings portray us as less like flaky dreamers, and more like folks who know what they are doing (though are still willing to learn), and aren't going to puke for a week straight or drive their captian to turn them into fish bait. Since our experience was as paid crew on tall ships, we also had professional references, which probably helped.
It is, however, extremely important to get to know each other as well as possible--email a lot, talk on the phone, get references. Not doing so can lead to very unpleasant situations. This danger is far less if you're just looking for crew/a boat for short trips. In that case, I don't see why a captain wouldn't occasionally be willing to take on inexperienced crew. If you're only sailing around the bay, what's the harm?
My boyfriend and I actually wrote a rather detailed article on our experiences and our strategies for finding boats to crew on. It will be coming out in the February 2013 edition of SAIL Magazine.
In conclusion: captians aren't exactly knocking down our door to have us on their boats, but, yes, crew finder forums work, if you're persistent and do it the right way.