I am thinking about registering for UX Marathon 2010 (http://www.uxmarathon.com) The price is affordable and there are Jared Spool, Luke Wroblewski and Rolf Molich (I've heard a lot great things about them) are among the speakers. I am just hesitating whether online conference is as useful as offline. Have you participated in something of that kind? Early registration is open only till October 13, so your opinions and advice are really welcome!
My wife did a Nielsen/Norman 3-day (5-day?) usability training and there was a lot of breaking into groups and doing practical exercises, prototyping UIs on paper, doing simple tests and so on. I don't think you'd get that in a webinar setting.
In general, I like webinars for information where it will be fairly passive, but I find that of course you can't do group work and I often find that it can be harder to have your voice heard. So I find it works great for lectures and interview style programming, but not so great for discussion-style programming.
But that's just me. There are so many learning styles. I tend to prefer learning to be either from reading or from highly interactive, face to face seminar style learning. I pretty much don't care for lecture style, whether live or via webinar. I know some people who are just the opposite.