I like panda's suggestion.

The hostess will not give out her number. If you leave a note, you've unnecessarily given yourself 2 more ways to lose.

Firstly, she will only call if she's really into you already, rather than just open to giving you a shot. If that were the case, she'd have made sure to leave her number the first night.

Secondly, the note-passer might screw up at some point. You have to rely on her willingness to help you out and do what's in your best interest. The note might not even reach her. You have no way of knowing whether it did. You can't ask the hostess twice. So the second contact attempt would have to be in person anyway. But this creates an awkward situation in case she did receive the note, but decided not to call.

I also like the touch of bringing another girl. Take the most attractive girl you can get to come with you. That way, you have a very solid plan B: sit there and enjoy a good dinner in the company of a nice girl. This saves you face, and adds a little 'I don't care if you like me or not, I've got other options' to your approach.

Of course, as mentioned above, these kind of elaborate schemes have a tendency to turn out different from the initial plan. Invariably this is because women react in unpredictable ways. Kind of reminds me of Seinfeld's menage a trois episode. But hey, that didn't turn out so bad, did it?