Kerry Finsand and the guys who launched Taplister are behind the app, and it works more or less the same as that site. I just downloaded it yesterday and I haven't screwed around with it much, but the functionality looks fantastic. Here's how Finsand and Co. describe it:
Beer Signal is an iPhone app that helps you bring your friends together to enjoy some good beer at your favorite establishments around town. Powered by Taplister.com, the app leverages the power of social networks like Twitter and Shizzow, and it simplifies the task of inviting the friends of your choice to join you at your current location, or to plan to meet somewhere in the future. By using the dynamic database of local pubs and watering holes, you can find who serves your favorite craft beers, or who has the latest seasonal brews on tap. You can also post a tweet telling your friends what you are drinking, or informing the database that a beer is no longer on tap at a particular location.It's free, so you should definitely download it if you have an iPhone. (To those of you who don't live in Portland: sorry. This version is only available for our fair city, though I assume they plan to expand in the future.) Let the informed tippling begin!
Update. In comments below, Patrick observes, "I tried Bailey's, which keeps an updated tap lists on their website, and the results did not correspond well at all."
I should have mentioned more about the function when I posted this item. It's truly an experiment in 21st Century connectivity and neural-networking. No one person keeps this updated (a task that would be nearly impossible). Rather, it depends on inputs from two main sources--Twitter and users. The designers have created a bot that updates the site when people or pubs use Twitter. (You have to use a specific nomenclature so the bot can find the updates: "drinking Pilsner Urquell at Green Dragon #ontappdx.") And now, with the app, you can instantly update taplists while you're drinking in a pub. The more people who use it and participate in keeping it updated, the more it will function properly. I've no doubt that the early weeks will be rocky, but I'm prepared to do my bit and keep it updated when I hit the pubs.