Agree with F-mattox...In fact, I just received one of those offers last week. Of course it sounds like all you have to do is attend a simple 90 minute presentation, but that is not the case. They will keep shifting the dates on you and there will also be a lot of upgrade charges, convenience fees and others and you do end up paying what you would have had you booked the trip yourself.
My advice it to try and get in contact with the cruise line and verify your dates. My guess is that they won't talk to you, but it is worth a try. I hope it works out for you, but I'll bet it doesn't. Call the company and verify the dates and see what they say. Then I would verify every few weeks and take detailed notes (and even record each conversation if it is legal in your state). Also, ALWAYS PAY BY CREDIT CARD AND NEVER BY CHECK!!!