Something a long these lines got me thinking about mail.
If you develop a list from your 'local bars & nightclubs' site it would behoove you to run some sort of marketing campaign featuring one of your listed bars. You could have them run some special that everyone who shows their email to the bouncer gets in for no cover or something like that. Everyone has iPhone/Palm/Blackberry/Android phones and can access their email. Have them pull the email up and that's their ticket in the door.
It would be a great way to demonstrate to the bar owner the power of marketing through your website.
Do it for nothing for a few times and get them hooked - then sit back while all the local bar owners bid up the price to be your exclusive Saturday evening drinkers alert email.
I saw something recently where a bar was offering free cover if you entered your phone number in their form and they sent a text message that was your ticket to get in. I don't think they were keeping the numbers to use in the future, but an opt in email for free cover sent via text would be great.