I'm working on something similar. My current approach is to create a separate product for each seminar and use simple stock levels (is that the proper name?) to limit attendance to each. for example, I have a product called "xyz seminar - 1 Jan 2008 through 1 Feb 2008". This works for me as I don't have a large number of seminars to handle. Your situation may be different.

I'm still researching other aspects of this, such as disabling shipping costs (but retaining a billing address), changing the language on the check-out screens to reflect that it's a seminar and not a physical thing, etc. I'm keen to hear how others have done it and whether there are any best practices to follow or gotchas to watch out for.