Ballsy, definitely, but think about the people who show up for that: you (if you're 37signals) know they'll sign up for anything you offer.

There's an important concept in sales called pre-qualification (i.e. only spend time pitching to prospects who are likely to buy) and this is one way of doing it, albeit an extreme one.

I think there is even more to it than that. Imagine the buzz that can be generated from 300 blogs going up after the conference about the latest, greatest 37Signals app coming up...