My first one was in a public place (library) so I wasn't nervous about that because I knew I could just turn up, have a look (at all the nappies on display) and then leave pretty much straight away if I didn't feel comfortable.
Conversely if you go to someones house, you don't feel like you can walk in, say 3 words and then leave if you feel a bit awkward. You are sort of compelled to say for a 'reasonable' length of time.
Of course, now that I do know lots of TNN Mums IRL, I don't hesitate to recommend nappiccinos to people (either public, or at someones house), because I've experienced first-hand what a great, interesting, non-threatening bunch of people TNNers are. I've even been game enough to go to a nappiccino in Auckland (and I didn't know anyone IRL from Auckland), and that was a BIG nappiccino.
Now I've just got to get organised and go to another one down here, because I haven't been for ages