Yes, it’s true, AMEX is doing a big promo with Foursquare for South by Southwest this year. Sign up and link your amex card with your Foursquare account, and get bonuses. That’s great for now, but is just a small first step in a much larger revolution – connecting our wallets together with our location, friends, and phones. Developments that may happen soon, as a result:

• Check in to a location simply by paying with your linked credit card
• Check in to a location simply by tapping your NFC-linked mobile phone, which is also linked to your location-feed account (Foursquare, Gowalla, etc), and also to your credit card
• PAY for goods by checking in and authorizing a payment. Is Foursquare the next Visa?!
• Pay for exclusive media content with your Foursquare account, and receive the download only when you are at the physical location. Bought tickets to that concert at Music Hall of Williamsburg with your AMEX? Check in when you get there, have Foursquare verify that you paid for tickets with your AMEX, and get a free download of the live concert recording, delivered over the 4G LTE network straight from the soundboard, as the curtains close.
• Bond together and get crowd-based ad-hoc discounts, based on number of users checked in at a location. At the Apple store with 5 of your friends? How about if all 5 of you check in to the same venue, and all buy a new iPod, paying with your linked Foursquare-Paypal account, receive an instant 10% discount for everybody. Instant, location based crowdsourced Groupon anybody?

Tabbed Out seems to be kicking off the trend:

TabbedOut is a secure and easy-to-use mobile payment application that allows patrons to pay their bar or restaurant tab with a smart phone—no more handing over a credit card to a server or bartender or leaving a credit card at the bar after a night on the town.